John:

This is an excellent, and important, query, and I will share with you the preliminary findings of a review of the issue I am doing [pending publication] to show that despite some stalling of Phase I trial results, some only - until recently - modest outcome benefits, and issues of both resistance and some disturbing hints of adverse antiapoptotic effects, nonetheless the interested and pace of research in TRAIL-based therapeutics is now firmly back on track, with some promising results from Phase II trials, and some highly hopeful new directions in all-biological therapies with a strong TRAIL component. I also include my own assessment of outstanding challenges, and some reflections and commentary on where this is all going, and some speculation as to how to proceed forward optimally.

TRAIL: PROMISE AND LIMITATIONS

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily, is one of the best-characterized apoptotic signaling pathways, interacting with its functional death receptors (DRs) and inducing apoptosis in a wide range of cancer cell types, and the activation of proapoptotic TRAIL receptors has long been considered an attractive promising approach in cancer therapeutics.

Against this "molecular" promise, and against the large body of preclinical (both in vitro and in vivo) data suggestive of potential benefit, TRAIL-based proapoptotic activation has however shown only highly modest positive results in human clinical trials, in part secondary to well-documented de novo or acquired resistance against TRAIL-driven apoptosis in many malignancies, and in part due to failures of patient selection in identifying the optimal target population (see below).

HUMAN CLINICAL DATA

TRAIL receptor-based therapies have been tested in a variety of malignancies. In colorectal cancer (CRC), a randomized phase 1b/2 trial of the TRAIL receptor agonist conatumumab when added to a FOLFOX plus bevacizumab (Avastin) combination regimen in the first-line treatment of metastatic CRC (see below), found no improved efficacy benefit over treatment without the inclusion of the TRAIL receptor agonist. In addition, also in the CRC context, a Phase II trial [17] of the TRAIL-R1 mAb agonist mapatumumab failed to achieve any objective response, and although 12 patients (32%) achieved stable disease, this was on a non-RECIST-compliant schedule of a median of 2.6 months, not > 6 months.

And in pediatric malignancies where such therapies have been widely tested, as pointed out by Marcia Moss in her contribution above, a recent trial [2] represents the results of the first pediatric Phase-I trial of lexatumumab in patients with various solid tumors, but the findings were similar to two other adult Phase-I studies done with this agent previously [3,4], and in which, like this trial, there were only hints of activity although no objective responses from this single agent in a Phase-I clinical setting.

However, more positively, in the lung cancer arena, the results of the first phase I trial of rhTRAIL (rhApo2L/ TRAIL) agent dulanermin were reported at the ASCO 2006 [5], showing that the monotherapy of rhTRAIL was well-tolerated and associated with some objective (partial) responses in patients with advanced solid cancer. A Phase Ib trial [6] then revealed that the combination of rhTRAIL with paclitaxel, carboplatin, and bevacizumab (PCB) improved the progression free survival (PFS) of patients with advanced NSCLC, and that of 18 patients, 10 experienced a confirmed response (9 partial and 1 complete) yielding a response rate of 56%. But in a Phase II clinical trial [24] of the same combination, it failed to improve efficacy outcomes in unselected patients with advanced NSCLC.

Note however that these were Phase I trials, with all the inherent limitations and weaknesses thereof, so it is telling that in contrast, the data from phase II trial of rhTRAIL was disappointing: the phase II trial [7] of 213 patients with advanced NSCLC revealed that the addition of PCB to rhTRAIL did not improve the progression-free survival of patients. Although some confounding factors such as small sample size and lack of control in the Phase Ib may partially explain the difference results of the Phase Ib and Phase II trials, the phase II trial in a large group of patients compels the conclusion that advanced NSCLCs are significantly, and perhaps intrinsically, resistant to recombinant human TRAIL (rhTRAIL) treatment.

But there is a second approach to targeting the TRAIL apoptotic pathway, namely by its agonistic monoclonal antibody (MAb) against DR4 and DR5. In this connection, the first time the generation of fully human agonistic MAbs against DR4, known as HGS-ETR1 or more commonly, mapatumumab, and against DR5 via HGS-ETR2 (commonly, lexatumumab), was reported at AACR 2002. A number of Phase I trials have evaluated the safety and pharmacokinetics of mapatumumab (HGS-ETR1), lexatumumab (HGS-ERT2) and another agent in this class, PRO95780 (drozitumab), a fully human DR5 MAb. But here two the early enthusiasm of Phase I trial findings were refuted at the Phase II trial level [8] at ASCO 2010, where the results from large groups of patients demonstrated that the addition of PCB or PC (paclitaxel, carboplatin) to PRO95780 and mapatumumab failed to achieve any objective response nor improved responsivity in patients with advanced cancer, once again demonstrating the resistance of advanced NSCLCs to the treatment of the TRAIL target agents.

In the lymphoma context, a Phase 1b clinical trial [9] was conducted with a combination of rituximab (anti-CD20, commercially Rituxan) plus rhTRAIL on patients with non Hodgkin's low grade lymphoma/NHL, finding that of 7 patients, 2 had a complete response, 1 a partial response and 2 some stabilization of the disease. However this was again a Phase I trial in NHL, and a Phase II trial of another TRAIL agent, PRO95780, found no benefit when added to rituximab, over rituximab alone [23].

Thus, it can be generalized that Phase I trials of single TRAIL receptor-based agents have been conducted on patients with advanced solid tumors and include the soluble rhTRAIL agent dulanermin [10], the TRAIL-R1 mAb agonist mapatumumab [11], and the TRAIL-R2 mAb agonists tigatuzumab [12], lexatumumab [4], and Apomab [13]. But although tolerability was generally good, true anticancer objective response was disappointing, with the vast majority of patients showing no regression of disease.

To date from my review and critical appraisal, the most promising TRAIL-targeting monotherapy has been mapatumumab which in a Phase II clinical trial [14] in a challenging population of patients with relapsed or refractory non-Hodgkin lymphoma (NHL) found that almost one-third of patients achieved true objective response, even with one showing complete recovery/response (CR), confirming the previous Phase I findings [11,15]. And in another trial, albeit Phase I [16], five patients (19%) achieved a confirmed radiologic partial response (rPR) , one of which was a pathologic complete response (pCR), with 12 patients (44%) showing stable disease as their best response. I note however that only seven patients (26%) had SD for ≥ 4 months, and even this falls short of the RECIST criteria of >=6 months. Nonetheless, the Phase II trial results are highly promising and we await further positive findings from in-progress trials.

From these modest results of TRAIL-targeting agents when used as monotherapy, it has then seemed attractive to move to combination regimens, such as the Phase II trial [7] of rhTRAIL (dulanermin) combined with paclitaxel, carboplatin, and bevacizumab, and other combination trials [1,9,16] which may be more likely to overcome resistance.

CHALLENGES REMAINING, AND LOOKING FORWARD

RESISTANCE

The main limit of clinical use of TRAIL-based therapies remains the innate or acquired resistance mechanisms: cells derived from many human cancers, such as colorectal cancer (CRC) and gliomas, and likely also NSCLC, are resistant against TRAIL-driven apoptosis due to defects in the TRAIL signaling machinery (e.g., down-regulation and/or impaired functionality of TRAIL receptors, increased level of anti-apoptotic proteins) [18], mainly due to reduced expression of death receptors (DRs) and/or up-regulation of TRAIL pathway-related anti-apoptotic proteins. Although as many observers have noted, combination therapies assist in overcoming such resistance mechanisms, I must note that this represent a considerable dilemma and limitation, since the initial attraction was to develop a TRAIL-based therapy that was targeted only to malignant cells, yet if traditional cytotoxics must be incorporated concurrently to overcome resistance, the clinical benefit TRAIL-based therapies becomes highly restricted, and can be challenged by other non-TRAIL-based strategies.

Nonetheless, some early results of "chemo-TRAIL" combination regimens do suggest some non-trivial promise: conatumumab, an agonistic monoclonal antibody against human death receptor 5 (DR5) achieved some objective responses in a Phase II trial [21] when used in combination with FOLFIRI for second-line treatment of mutant KRAS metastatic colorectal cancer, a challenging population, and the same agent (conatumumab) showed some evidence of activity as assessed by the 6-month survival rate, when used in combination with gemcitabine in patients with metastatic pancreatic cancer [22].

OPEN ISSUES

So there are several questions which needs to be comprehensively addressed before moving forward.

(1) What’s the most optimal patient population who might benefit?

(2) What’s the best modality for combination therapy to minimize toxicity but still gain some appreciable benefit from the TRAIL-based treatment, were the choices are chemotherapy, or radiotherapy, or biological therapy, and which in what combination to maximize survival outcomes?

(3) What are the relative benefits of TRAIL-based therapies compared to other new and emerging paradigm, that is what are their true place in the new treatment paradigms?

GOING FORWARD

From preliminary results, it would appear from this review that ideally, we would want to focus research efforts on biological therapies concurrent with TRAIL-based agents, for these reasons:

(1) unlike with chemotherapies which reintroduce the problems of high toxicity and low tolerability, thus in a sense defeating the reason d'être of TRAIL targeting which was attractive in particular for its lack of effect on normal cells, being malignant-cell-specific, biological therapies avoid cytotoxicities.

(2) we have two promising biological classes that these preliminary results suggest can increase the expression of TRAIL receptors DR5 and/or DR4, reduce the levels of c-FLIP which may help overcome resistance, and enhance TRAIL induced apoptosis, namely HDAC/DNMT inhibitors and Proteasome inhibitors, now undergoing further research and clinical trial testing.

(3) given that although the benefits of TRAIL therapy are disappointingly modest overall, nonetheless there appears to be a small subset of patients who are highly responsive to TRAIL, this suggest that we are likely to realize and exploit the true potential of targeting TRAIL DRs only when we learn how to select for those TRAIL-responsive patients most likely to accrue significant benefit from the treatment, and for this to happen, it is essential to identify biomarkers that can help us predict TRAIL sensitivity, with efforts already achieving some provisional promising results towards that goal [19].

Indeed, from my review it is clear that there has been only limited translatability from encouraging preclinical studies ("bench") into the clinic ("bedside"), and I draw attention to the fact that in virtually all of the failed trial or those with only a hint of potential benefit, none of these trials preselected patients on the basis of degree of TRAIL sensitivity,, so we must develop robust tests for such sensitivity, and its identification through TRAIL-specific biomarkers of responsivity.

A final challenge remains, often overlooked: we have evidence that signaling through TRAIL death receptors may also induce antiapoptotic, not proapoptotic, responses and encourage proliferation signaling: this was seen in subpopulations of primary acute leukemia cells isolated from pediatric patients which showed increased proliferation rates after TRAIL treatment [20], showing TRAIL cancer dynamics to be far more complex than previously realized. This, it strikes me, can best be overcome by coupling TRAIL agents into combination regimens with biological agents that are selected to overcome these prosurvival tumor responses, and this further suggests that the usability of TRAIL monotherapy is simple unlikely to prove clinically viable, as as indicated above the biologics with the greatest promise for countervailing antiapoptotic response from TRAIL are, as noted above, (1) HDAC/DNMT inhibitors, and (2) Proteasome inhibitors. I am awaiting some hopeful results for these all-biological regimens in the not too distant future.

METHODOLOGY OF THIS REVIEW

A search of the PUBMED, Cochrane Library / Cochrane Register of Controlled Trials, MEDLINE, EMBASE, AMED (Allied and Complimentary Medicine Database), CINAHL (Cumulative Index to Nursing and Allied Health Literature), PsycINFO, ISI Web of Science (WoS), BIOSIS, LILACS (Latin American and Caribbean Health Sciences Literature), ASSIA (Applied Social Sciences Index and Abstracts), SCEH (NHS Evidence Specialist Collection for Ethnicity and Health) and SCIRUS databases was conducted without language or date restrictions, and updated again current as of date of publication, with systematic reviews and meta-analyses extracted separately. Search was expanded in parallel to include just-in-time (JIT) medical feed sources as returned from Terkko (provided by the National Library of Health Sciences - Terkko at the University of Helsinki). A further "broad-spectrum" science search using SCIRUS (410+ million entry database) was then deployed for resources not otherwise included. Unpublished studies were located via contextual search, and relevant dissertations were located via NTLTD (Networked Digital Library of Theses and Dissertations) and OpenThesis. Sources in languages foreign to this reviewer were translated by language translation software.

REFERENCES

1. Fuchs CS, Fakih M, Schwartzberg L, et al. TRAIL receptor agonist conatumumab with modified FOLFOX6 plus bevacizumab for first-line treatment of metastatic colorectal cancer: A randomized phase 1b/2 trial. Cancer 2013 Oct 1.

2. Merchant MS, Geller JI, Baird K, et al. Phase I trial and pharmacokinetic study of lexatumumab in pediatric patients with solid tumors. J Clin Oncol 2012;30:4141-7.

3. Plummer R, Attard G, Pacey S, et al. Phase 1 and pharmacokinetic study of lexatumumab in patients with advanced cancers. Clin Cancer Res 2007;13:6187-94.

4. Wakelee HA, Patnaik A, Sikic BI, et al. Phase I and pharmacokinetic study of lexatumumab (HGS-ETR2) given every 2 weeks in patients with advanced solid tumors. Ann Oncol 2010;21:376-81.

5. Herbst RS, Eckhardt SG, Kurzrock R, et al. Phase I dose-escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J Clin Oncol 2010;28:2839-46.

6. Soria JC, Smit E, Khayat D, et al. Phase 1b study of dulanermin (recombinant human Apo2L/TRAIL) in combination with paclitaxel, carboplatin, and bevacizumab in patients with advanced non-squamous non-small-cell lung cancer. J Clin Oncol 2010;28:1527-33.

7. Soria JC, Márk Z, Zatloukal P, et al. Randomized phase II study of dulanermin in combination with paclitaxel, carboplatin, and bevacizumab in advanced non-small-cell lung cancer. J Clin Oncol 2011;29:4442-51.

8. Camidge DR, Herbst RS, Gordon MS, et al. A phase I safety and pharmacokinetic study of the death receptor 5 agonistic antibody PRO95780 in patients with advanced malignancies. Clin Cancer Res 2010;16:1256-63.

9. Yee L, Fanale M, Dimick K, Calvert S, Robins C, Ing J, Ling J, Novotny W, Ashkenazi A, Burris I,H. A phase IB safety and pharmacokinetic (PK) study of recombinant human Apo2L/TRAIL in combination with rituximab in patients with low-grade non-Hodgkin lymphoma. J. Clin. Oncol. 2007;25:8078.

10. Herbst, R.S. et al. (2010) Phase I dose–escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J. Clin. Oncol. 28, 2839–2846.

11. Hotte, S.J. et al. (2008) A phase 1 study of mapatumumab (fully human monoclonal antibody to TRAIL-R1) in patients with advanced solid malignancies. Clin. Cancer Res. 14, 3450–3455.

12. Forero-Torres, A. et al. (2010) Phase I trial of weekly tigatuzumab, an agonistic humanized monoclonal antibody targeting death receptor 5 (DR5). Cancer Biother. Radiopharm. 25, 13–19.

13. Camidge, D.R. (2008) Apomab: an agonist monoclonal antibody directed against Death Receptor 5/TRAIL-Receptor 2 for use in the treatment of solid tumors. Expert Opin. Biol. Ther. 8, 1167–1176.

14. Younes, A. et al. (2010) A Phase 1b/2 trial of mapatumumab in patients with relapsed/refractory non-Hodgkin’s lymphoma. Br J Cancer 103, 1783–1787.

15. Tolcher AW, Mita M, Meropol NJ, von Mehren M, Patnaik A, Padavic K, et al. Phase I pharmacokinetic and biologic correlative study of mapatumumab, a fully human monoclonal antibody with agonist activity to tumor necrosis factor-related apoptosis-inducing ligand receptor-1. J Clin Oncol 2007;25:1390–5.

16. Leong S, Cohen RB, Gustafson DL, et al. Mapatumumab, an antibody targeting TRAIL-R1, in combination with paclitaxel and carboplatin in patients with advanced solid malignancies: results of a phase I and pharmacokinetic study. J Clin Oncol 2009 Sep 10; 27(26):4413-21.

17. Trarbach T, Moehler M, Heinemann V, et al. Phase II trial of mapatumumab, a fully human agonistic monoclonal antibody that targets and activates the tumour necrosis factor apoptosis-inducing ligand receptor-1 (TRAIL-R1), in patients with refractory colorectal cancer. Br J Cancer 2010 Feb 2; 102(3):506-12.

18. Zhang, L.; Fang, B. Mechanisms of resistance to trail-induced apoptosis in cancer. Cancer Gene Ther. 2005, 12, 228–237.

19. Dimberg LY, Anderson CK, Camidge R, Behbakht K, Thorburn A, Ford HL. On the TRAIL to successful cancer therapy? Predicting and counteracting resistance against TRAIL-based therapeutics. Oncogene 2013 Mar 14; 32(11):1341-50.

20. Ehrhardt H, Fulda S, Schmid I, Hiscott J, Debatin KM, Jeremias I. TRAIL induced survival and proliferation in cancer cells resistant towards TRAIL-induced apoptosis mediated by NF-kappaB. Oncogene 2003;22:3842–52.

21. Cohn AL, Tabernero J, Maurel J, et al. A randomized, placebo-controlled phase 2 study of ganitumab or conatumumab in combination with FOLFIRI for second-line treatment of mutant KRAS metastatic colorectal cancer. Ann Oncol 2013; 24(7):1777-85.

22. Kindler HL, Richards DA, Garbo LE, et al. A randomized, placebo-controlled phase 2 study of ganitumab (AMG 479) or conatumumab (AMG 655) in combination with gemcitabine in patients with metastatic pancreatic cancer. Ann Oncol 2012; 23(11):2834-42.

23. Wittebol S, Ferrant A, Wickham NW, Fehrenbacher L, Durbin-Johnson B, Bray GL. Phase II study of PRO95780 plus rituximab in patients with relapsed follicular non-Hodgkin's lymphoma (NHL). J Clin Oncol (ASCO Meeting Abstracts) May 2010 vol. 28 no. 15_suppl e18511.

24. Karapetis CS, Clingan PR, Leighl NB, Durbin-Johnson B, O'Neill V, Spigel DR. Phase II study of PRO95780 plus paclitaxel, carboplatin, and bevacizumab (PCB) in non-small cell lung cancer (NSCLC). J Clin Oncol (ASCO Meeting Abstracts) May 2010 vol. 28 no. 15_suppl 7535.

Constantine Kaniklidis

Director of Medical Research

No Surrender Breast Cancer Foundation (NSBCF)

There was an anti trail 2 receptor clinical trial report, Merchant et al, J. Clin. Oncology, in 2012 in pediatric patients. Did not see any other clinical trails, other than 1 in 2006.

I did not look up the reference. I am interested in this because ADAM inhibitors affect TRAIL receptor shedding. I would think that an ADAM inhibitor would increase the responsiveness, of other therapies that act through TRAIL receptors.

John:

This is an excellent, and important, query, and I will share with you the preliminary findings of a review of the issue I am doing [pending publication] to show that despite some stalling of Phase I trial results, some only - until recently - modest outcome benefits, and issues of both resistance and some disturbing hints of adverse antiapoptotic effects, nonetheless the interested and pace of research in TRAIL-based therapeutics is now firmly back on track, with some promising results from Phase II trials, and some highly hopeful new directions in all-biological therapies with a strong TRAIL component. I also include my own assessment of outstanding challenges, and some reflections and commentary on where this is all going, and some speculation as to how to proceed forward optimally.

TRAIL: PROMISE AND LIMITATIONS

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily, is one of the best-characterized apoptotic signaling pathways, interacting with its functional death receptors (DRs) and inducing apoptosis in a wide range of cancer cell types, and the activation of proapoptotic TRAIL receptors has long been considered an attractive promising approach in cancer therapeutics.

Against this "molecular" promise, and against the large body of preclinical (both in vitro and in vivo) data suggestive of potential benefit, TRAIL-based proapoptotic activation has however shown only highly modest positive results in human clinical trials, in part secondary to well-documented de novo or acquired resistance against TRAIL-driven apoptosis in many malignancies, and in part due to failures of patient selection in identifying the optimal target population (see below).

HUMAN CLINICAL DATA

TRAIL receptor-based therapies have been tested in a variety of malignancies. In colorectal cancer (CRC), a randomized phase 1b/2 trial of the TRAIL receptor agonist conatumumab when added to a FOLFOX plus bevacizumab (Avastin) combination regimen in the first-line treatment of metastatic CRC (see below), found no improved efficacy benefit over treatment without the inclusion of the TRAIL receptor agonist. In addition, also in the CRC context, a Phase II trial [17] of the TRAIL-R1 mAb agonist mapatumumab failed to achieve any objective response, and although 12 patients (32%) achieved stable disease, this was on a non-RECIST-compliant schedule of a median of 2.6 months, not > 6 months.

And in pediatric malignancies where such therapies have been widely tested, as pointed out by Marcia Moss in her contribution above, a recent trial [2] represents the results of the first pediatric Phase-I trial of lexatumumab in patients with various solid tumors, but the findings were similar to two other adult Phase-I studies done with this agent previously [3,4], and in which, like this trial, there were only hints of activity although no objective responses from this single agent in a Phase-I clinical setting.

However, more positively, in the lung cancer arena, the results of the first phase I trial of rhTRAIL (rhApo2L/ TRAIL) agent dulanermin were reported at the ASCO 2006 [5], showing that the monotherapy of rhTRAIL was well-tolerated and associated with some objective (partial) responses in patients with advanced solid cancer. A Phase Ib trial [6] then revealed that the combination of rhTRAIL with paclitaxel, carboplatin, and bevacizumab (PCB) improved the progression free survival (PFS) of patients with advanced NSCLC, and that of 18 patients, 10 experienced a confirmed response (9 partial and 1 complete) yielding a response rate of 56%. But in a Phase II clinical trial [24] of the same combination, it failed to improve efficacy outcomes in unselected patients with advanced NSCLC.

Note however that these were Phase I trials, with all the inherent limitations and weaknesses thereof, so it is telling that in contrast, the data from phase II trial of rhTRAIL was disappointing: the phase II trial [7] of 213 patients with advanced NSCLC revealed that the addition of PCB to rhTRAIL did not improve the progression-free survival of patients. Although some confounding factors such as small sample size and lack of control in the Phase Ib may partially explain the difference results of the Phase Ib and Phase II trials, the phase II trial in a large group of patients compels the conclusion that advanced NSCLCs are significantly, and perhaps intrinsically, resistant to recombinant human TRAIL (rhTRAIL) treatment.

But there is a second approach to targeting the TRAIL apoptotic pathway, namely by its agonistic monoclonal antibody (MAb) against DR4 and DR5. In this connection, the first time the generation of fully human agonistic MAbs against DR4, known as HGS-ETR1 or more commonly, mapatumumab, and against DR5 via HGS-ETR2 (commonly, lexatumumab), was reported at AACR 2002. A number of Phase I trials have evaluated the safety and pharmacokinetics of mapatumumab (HGS-ETR1), lexatumumab (HGS-ERT2) and another agent in this class, PRO95780 (drozitumab), a fully human DR5 MAb. But here two the early enthusiasm of Phase I trial findings were refuted at the Phase II trial level [8] at ASCO 2010, where the results from large groups of patients demonstrated that the addition of PCB or PC (paclitaxel, carboplatin) to PRO95780 and mapatumumab failed to achieve any objective response nor improved responsivity in patients with advanced cancer, once again demonstrating the resistance of advanced NSCLCs to the treatment of the TRAIL target agents.

In the lymphoma context, a Phase 1b clinical trial [9] was conducted with a combination of rituximab (anti-CD20, commercially Rituxan) plus rhTRAIL on patients with non Hodgkin's low grade lymphoma/NHL, finding that of 7 patients, 2 had a complete response, 1 a partial response and 2 some stabilization of the disease. However this was again a Phase I trial in NHL, and a Phase II trial of another TRAIL agent, PRO95780, found no benefit when added to rituximab, over rituximab alone [23].

Thus, it can be generalized that Phase I trials of single TRAIL receptor-based agents have been conducted on patients with advanced solid tumors and include the soluble rhTRAIL agent dulanermin [10], the TRAIL-R1 mAb agonist mapatumumab [11], and the TRAIL-R2 mAb agonists tigatuzumab [12], lexatumumab [4], and Apomab [13]. But although tolerability was generally good, true anticancer objective response was disappointing, with the vast majority of patients showing no regression of disease.

To date from my review and critical appraisal, the most promising TRAIL-targeting monotherapy has been mapatumumab which in a Phase II clinical trial [14] in a challenging population of patients with relapsed or refractory non-Hodgkin lymphoma (NHL) found that almost one-third of patients achieved true objective response, even with one showing complete recovery/response (CR), confirming the previous Phase I findings [11,15]. And in another trial, albeit Phase I [16], five patients (19%) achieved a confirmed radiologic partial response (rPR) , one of which was a pathologic complete response (pCR), with 12 patients (44%) showing stable disease as their best response. I note however that only seven patients (26%) had SD for ≥ 4 months, and even this falls short of the RECIST criteria of >=6 months. Nonetheless, the Phase II trial results are highly promising and we await further positive findings from in-progress trials.

From these modest results of TRAIL-targeting agents when used as monotherapy, it has then seemed attractive to move to combination regimens, such as the Phase II trial [7] of rhTRAIL (dulanermin) combined with paclitaxel, carboplatin, and bevacizumab, and other combination trials [1,9,16] which may be more likely to overcome resistance.

CHALLENGES REMAINING, AND LOOKING FORWARD

RESISTANCE

The main limit of clinical use of TRAIL-based therapies remains the innate or acquired resistance mechanisms: cells derived from many human cancers, such as colorectal cancer (CRC) and gliomas, and likely also NSCLC, are resistant against TRAIL-driven apoptosis due to defects in the TRAIL signaling machinery (e.g., down-regulation and/or impaired functionality of TRAIL receptors, increased level of anti-apoptotic proteins) [18], mainly due to reduced expression of death receptors (DRs) and/or up-regulation of TRAIL pathway-related anti-apoptotic proteins. Although as many observers have noted, combination therapies assist in overcoming such resistance mechanisms, I must note that this represent a considerable dilemma and limitation, since the initial attraction was to develop a TRAIL-based therapy that was targeted only to malignant cells, yet if traditional cytotoxics must be incorporated concurrently to overcome resistance, the clinical benefit TRAIL-based therapies becomes highly restricted, and can be challenged by other non-TRAIL-based strategies.

Nonetheless, some early results of "chemo-TRAIL" combination regimens do suggest some non-trivial promise: conatumumab, an agonistic monoclonal antibody against human death receptor 5 (DR5) achieved some objective responses in a Phase II trial [21] when used in combination with FOLFIRI for second-line treatment of mutant KRAS metastatic colorectal cancer, a challenging population, and the same agent (conatumumab) showed some evidence of activity as assessed by the 6-month survival rate, when used in combination with gemcitabine in patients with metastatic pancreatic cancer [22].

OPEN ISSUES

So there are several questions which needs to be comprehensively addressed before moving forward.

(1) What’s the most optimal patient population who might benefit?

(2) What’s the best modality for combination therapy to minimize toxicity but still gain some appreciable benefit from the TRAIL-based treatment, were the choices are chemotherapy, or radiotherapy, or biological therapy, and which in what combination to maximize survival outcomes?

(3) What are the relative benefits of TRAIL-based therapies compared to other new and emerging paradigm, that is what are their true place in the new treatment paradigms?

GOING FORWARD

From preliminary results, it would appear from this review that ideally, we would want to focus research efforts on biological therapies concurrent with TRAIL-based agents, for these reasons:

(1) unlike with chemotherapies which reintroduce the problems of high toxicity and low tolerability, thus in a sense defeating the reason d'être of TRAIL targeting which was attractive in particular for its lack of effect on normal cells, being malignant-cell-specific, biological therapies avoid cytotoxicities.

(2) we have two promising biological classes that these preliminary results suggest can increase the expression of TRAIL receptors DR5 and/or DR4, reduce the levels of c-FLIP which may help overcome resistance, and enhance TRAIL induced apoptosis, namely HDAC/DNMT inhibitors and Proteasome inhibitors, now undergoing further research and clinical trial testing.

(3) given that although the benefits of TRAIL therapy are disappointingly modest overall, nonetheless there appears to be a small subset of patients who are highly responsive to TRAIL, this suggest that we are likely to realize and exploit the true potential of targeting TRAIL DRs only when we learn how to select for those TRAIL-responsive patients most likely to accrue significant benefit from the treatment, and for this to happen, it is essential to identify biomarkers that can help us predict TRAIL sensitivity, with efforts already achieving some provisional promising results towards that goal [19].

Indeed, from my review it is clear that there has been only limited translatability from encouraging preclinical studies ("bench") into the clinic ("bedside"), and I draw attention to the fact that in virtually all of the failed trial or those with only a hint of potential benefit, none of these trials preselected patients on the basis of degree of TRAIL sensitivity,, so we must develop robust tests for such sensitivity, and its identification through TRAIL-specific biomarkers of responsivity.

A final challenge remains, often overlooked: we have evidence that signaling through TRAIL death receptors may also induce antiapoptotic, not proapoptotic, responses and encourage proliferation signaling: this was seen in subpopulations of primary acute leukemia cells isolated from pediatric patients which showed increased proliferation rates after TRAIL treatment [20], showing TRAIL cancer dynamics to be far more complex than previously realized. This, it strikes me, can best be overcome by coupling TRAIL agents into combination regimens with biological agents that are selected to overcome these prosurvival tumor responses, and this further suggests that the usability of TRAIL monotherapy is simple unlikely to prove clinically viable, as as indicated above the biologics with the greatest promise for countervailing antiapoptotic response from TRAIL are, as noted above, (1) HDAC/DNMT inhibitors, and (2) Proteasome inhibitors. I am awaiting some hopeful results for these all-biological regimens in the not too distant future.

METHODOLOGY OF THIS REVIEW

A search of the PUBMED, Cochrane Library / Cochrane Register of Controlled Trials, MEDLINE, EMBASE, AMED (Allied and Complimentary Medicine Database), CINAHL (Cumulative Index to Nursing and Allied Health Literature), PsycINFO, ISI Web of Science (WoS), BIOSIS, LILACS (Latin American and Caribbean Health Sciences Literature), ASSIA (Applied Social Sciences Index and Abstracts), SCEH (NHS Evidence Specialist Collection for Ethnicity and Health) and SCIRUS databases was conducted without language or date restrictions, and updated again current as of date of publication, with systematic reviews and meta-analyses extracted separately. Search was expanded in parallel to include just-in-time (JIT) medical feed sources as returned from Terkko (provided by the National Library of Health Sciences - Terkko at the University of Helsinki). A further "broad-spectrum" science search using SCIRUS (410+ million entry database) was then deployed for resources not otherwise included. Unpublished studies were located via contextual search, and relevant dissertations were located via NTLTD (Networked Digital Library of Theses and Dissertations) and OpenThesis. Sources in languages foreign to this reviewer were translated by language translation software.

REFERENCES

1. Fuchs CS, Fakih M, Schwartzberg L, et al. TRAIL receptor agonist conatumumab with modified FOLFOX6 plus bevacizumab for first-line treatment of metastatic colorectal cancer: A randomized phase 1b/2 trial. Cancer 2013 Oct 1.

2. Merchant MS, Geller JI, Baird K, et al. Phase I trial and pharmacokinetic study of lexatumumab in pediatric patients with solid tumors. J Clin Oncol 2012;30:4141-7.

3. Plummer R, Attard G, Pacey S, et al. Phase 1 and pharmacokinetic study of lexatumumab in patients with advanced cancers. Clin Cancer Res 2007;13:6187-94.

4. Wakelee HA, Patnaik A, Sikic BI, et al. Phase I and pharmacokinetic study of lexatumumab (HGS-ETR2) given every 2 weeks in patients with advanced solid tumors. Ann Oncol 2010;21:376-81.

5. Herbst RS, Eckhardt SG, Kurzrock R, et al. Phase I dose-escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J Clin Oncol 2010;28:2839-46.

6. Soria JC, Smit E, Khayat D, et al. Phase 1b study of dulanermin (recombinant human Apo2L/TRAIL) in combination with paclitaxel, carboplatin, and bevacizumab in patients with advanced non-squamous non-small-cell lung cancer. J Clin Oncol 2010;28:1527-33.

7. Soria JC, Márk Z, Zatloukal P, et al. Randomized phase II study of dulanermin in combination with paclitaxel, carboplatin, and bevacizumab in advanced non-small-cell lung cancer. J Clin Oncol 2011;29:4442-51.

8. Camidge DR, Herbst RS, Gordon MS, et al. A phase I safety and pharmacokinetic study of the death receptor 5 agonistic antibody PRO95780 in patients with advanced malignancies. Clin Cancer Res 2010;16:1256-63.

9. Yee L, Fanale M, Dimick K, Calvert S, Robins C, Ing J, Ling J, Novotny W, Ashkenazi A, Burris I,H. A phase IB safety and pharmacokinetic (PK) study of recombinant human Apo2L/TRAIL in combination with rituximab in patients with low-grade non-Hodgkin lymphoma. J. Clin. Oncol. 2007;25:8078.

10. Herbst, R.S. et al. (2010) Phase I dose–escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J. Clin. Oncol. 28, 2839–2846.

11. Hotte, S.J. et al. (2008) A phase 1 study of mapatumumab (fully human monoclonal antibody to TRAIL-R1) in patients with advanced solid malignancies. Clin. Cancer Res. 14, 3450–3455.

12. Forero-Torres, A. et al. (2010) Phase I trial of weekly tigatuzumab, an agonistic humanized monoclonal antibody targeting death receptor 5 (DR5). Cancer Biother. Radiopharm. 25, 13–19.

13. Camidge, D.R. (2008) Apomab: an agonist monoclonal antibody directed against Death Receptor 5/TRAIL-Receptor 2 for use in the treatment of solid tumors. Expert Opin. Biol. Ther. 8, 1167–1176.

14. Younes, A. et al. (2010) A Phase 1b/2 trial of mapatumumab in patients with relapsed/refractory non-Hodgkin’s lymphoma. Br J Cancer 103, 1783–1787.

15. Tolcher AW, Mita M, Meropol NJ, von Mehren M, Patnaik A, Padavic K, et al. Phase I pharmacokinetic and biologic correlative study of mapatumumab, a fully human monoclonal antibody with agonist activity to tumor necrosis factor-related apoptosis-inducing ligand receptor-1. J Clin Oncol 2007;25:1390–5.

16. Leong S, Cohen RB, Gustafson DL, et al. Mapatumumab, an antibody targeting TRAIL-R1, in combination with paclitaxel and carboplatin in patients with advanced solid malignancies: results of a phase I and pharmacokinetic study. J Clin Oncol 2009 Sep 10; 27(26):4413-21.

17. Trarbach T, Moehler M, Heinemann V, et al. Phase II trial of mapatumumab, a fully human agonistic monoclonal antibody that targets and activates the tumour necrosis factor apoptosis-inducing ligand receptor-1 (TRAIL-R1), in patients with refractory colorectal cancer. Br J Cancer 2010 Feb 2; 102(3):506-12.

18. Zhang, L.; Fang, B. Mechanisms of resistance to trail-induced apoptosis in cancer. Cancer Gene Ther. 2005, 12, 228–237.

19. Dimberg LY, Anderson CK, Camidge R, Behbakht K, Thorburn A, Ford HL. On the TRAIL to successful cancer therapy? Predicting and counteracting resistance against TRAIL-based therapeutics. Oncogene 2013 Mar 14; 32(11):1341-50.

20. Ehrhardt H, Fulda S, Schmid I, Hiscott J, Debatin KM, Jeremias I. TRAIL induced survival and proliferation in cancer cells resistant towards TRAIL-induced apoptosis mediated by NF-kappaB. Oncogene 2003;22:3842–52.

21. Cohn AL, Tabernero J, Maurel J, et al. A randomized, placebo-controlled phase 2 study of ganitumab or conatumumab in combination with FOLFIRI for second-line treatment of mutant KRAS metastatic colorectal cancer. Ann Oncol 2013; 24(7):1777-85.

22. Kindler HL, Richards DA, Garbo LE, et al. A randomized, placebo-controlled phase 2 study of ganitumab (AMG 479) or conatumumab (AMG 655) in combination with gemcitabine in patients with metastatic pancreatic cancer. Ann Oncol 2012; 23(11):2834-42.

23. Wittebol S, Ferrant A, Wickham NW, Fehrenbacher L, Durbin-Johnson B, Bray GL. Phase II study of PRO95780 plus rituximab in patients with relapsed follicular non-Hodgkin's lymphoma (NHL). J Clin Oncol (ASCO Meeting Abstracts) May 2010 vol. 28 no. 15_suppl e18511.

24. Karapetis CS, Clingan PR, Leighl NB, Durbin-Johnson B, O'Neill V, Spigel DR. Phase II study of PRO95780 plus paclitaxel, carboplatin, and bevacizumab (PCB) in non-small cell lung cancer (NSCLC). J Clin Oncol (ASCO Meeting Abstracts) May 2010 vol. 28 no. 15_suppl 7535.

Constantine Kaniklidis

Director of Medical Research

No Surrender Breast Cancer Foundation (NSBCF)

Wow, what an answer. Constantine. I have a general question. Do the trail drug candidates work well alone or better in combination in typical animal models and cell based assays. As for trail signalling either being pro apoptotic or anti apoptotic, this finding is similar with TGF beta signalling. When TGFBRIII is present, the signalling works to stop cancer growth. When the TGFBRIII is absent, the chemo doesn't work.

The TGFBRIII needs to be soluble. So maybe this is true with the trail receptors, ie it is the balance of the receptors as being in the soluble or membrane bound form. This is important to know because when setting up a clinical trial, one May be able to predict the responders which would help in the success of the trial. Maybe understanding why trail is pro apoptosis or apototic, will help.

I meant to write, maybe understanding what signalling pathways are activated or turned off with the trail drug candidates will help to determine why in one case they are pro apoptosis and in another case anti apoptosis. And you can change the balance between soluble and membrane bound forms with ADAM inhibitors.

Marcia, Eswar:

Marcia makes some excellent observations and raises important questions, so let me try to address the issues and place them in a wider clinical context. And Eswar draws worthwhile attention to the disparity between the over-enthusiastic promise of early in vitro studies and the later reality of rather modest benefits in the human clinical context, largely secondary to the problem of de novo or acquired TRAIL resistance.

However, let me note that we are in a second "era" of TRAIL research (begun roughly 2012 and continued with accelerated activity throughout 2013 (some cited below, and previously), and with some more positive results in several dozens publications and clinical trials I am reviewing which are pending publication but not yet available to the general oncology community. In this second-generation of TRAIL research, selective synergies are being successfully used to overcome TRAIL resistance through innovative "partners" coupled to TRAIL-based agents, and pending confirmation in human clinical trials, we should be beginning to see some of the theoretical promise of preclinical data translate into real clinical benefits on the "bedside" front of the equation. And for that we still require biomarkers or gene expression signatures - many now identified - that can determine which is the dominating resistance pathway in a particular patient with a particular malignancy type, in order to discover the optimally safe and efficacious "TRAIL-combinatorics" (with the greatest promise, as I noted above, stemming from biological combinations with Proteasome inhibitors, and I think even more with HDAC/DNMT inhibitors (including natural agents). But as I note below, and previously, challenges still remain, and we will need cross-confirmatory positive findings from robust Phase II or especially Phase III clinical trials in addition to strong observational results before TRAIL-based interventions take their place in the current and emerging therapeutic anticancer arsenal.

A BRIEF PRIMER ON TRAIL THERAPEUTIC DYNAMICS

So, first, it is certainly now well-established that the anti-apoptotic activity of TRAIL monotherapy is far too modest to be of clinical relevance, and enables a sanctuary for the development of TRAIL resistance, so only combination regimens are likely to exert sufficient aggregate benefit to positively influence both responsivity and survival outcome. Second, we now have - see below - some fairly comprehensive information as to the molecular pathways influencing the activation of anti-apoptotic mechanisms and the down-regulation of pro-apoptotic mechanisms (which as noted below tend to happen concurrently to evade TRAIL-mediated apoptosis). Indeed, recent research has shown that inhibition/knockdown of anti-apoptotic proteins is sufficient to induce TRAIL sensitivity, and that increased expression of pro-apoptotic proteins would generate the same effect, and more importantly, most tumor cell types rely on the expression of a just a single anti-apoptotic protein, so that the inhibition of this pivotal blockade is sufficient to re-activate TRAIL sensitivity [16], although unsurprisingly which anti-apoptotic protein is involved is malignancy-dependent and is typically not unitary (several different such proteins can produce the same effect in some particular malignancy. Thus, in melanoma, knockdown or inhibition of cFLIP alone [17] is sufficient to restore TRAIL sensitivity, while the same inhibition of XIAP overcomes TRAIL resistance in pancreatic cancer cells [18], while in others knockdown or inhibition of the Bcl-2 block (including Mcl-1) is sufficient to sensitize several cancer cell types to TRAIL-induced apoptosis (see my discussion of Mcl-1 below).

sTRAIL (SOLUBLE TRAIL)

Third, re the observation (Marcia) concerning soluble TRAIL, I note that, among others, Pamela Holland with Amgen [10], based on earlier preclinical data [11,12] has most forcibly posited targeting TRAIL receptors by using soluble Apo2L/TRAIL, known as "sTRAIL", and which is generated by deletion of the transmembrane and intracellular domains, and hence is essentially a non-membrane-anchored bindable activation target, and there is converging evidence that sTRAIL induces apoptosis in, among others, lung cancer cells through TRAIL-R2 (DR5) [13]. But to date the exact role and place of sTRAIL compared to membrane-TRAIL remains unresolved, as does the issue of whether sTRAIL interventions offer any clinically relevant advantages over membrane-TRAIL interventions (given that for instance that for TRAIL-R2 to be activated by soluble TRAIL requires the latter to be secondarily cross-linked by antibodies, but in contrast TRAIL-R2 is activated directly by membrane-TRAIL with no assistive antibody-based cross-linking [12].

EMBARASSMENT OF RICHES

Over a hundred mechanisms and associated biomarkers too numerous to mention have been proposed and tested for TRAIL sensitization, ranging from gene signatures of TRAIL-sensitivity [1], the multidrug transporter P-glycoprotein (Pgp) whose expression appears to confer resistance to TRAIL, while blocking Pgp transport activity sensitizes the malignant cells toward TRAIL at least within the domain of HeLa cells [2], PTHrP overexpression (with mediation from Akt/MEK and other related molecular pathways) as TRAIL-sensitizing within the breast cancer context [3], K-Ras mutations in pancreatic cancers [4], out to the microRNA miR-130a in NSCLC [5], and so on for a numbingly long list of mechanisms and biomarkers, most malignancy-specific, with little attention paid to underlying and potentially communal integrating processes and pathways.

TOWARDS A UNIFORM THEORY

But some progress is being made in this direction of a "uniform theory" of TRAIL sensitization. It all begins with the realization that ligands of the TNF family all initiate the extrinsic apoptotic pathway through binding to cell surface death receptors (DRs) of the TNF receptor superfamily, and this engagement of the death receptors ultimately leads to the formation of the death-inducing signaling complex, and finally to the activation of the initiator caspase-8 (via an intermediate activation of caspase-3). But unlike in type I cell surface receptors (RI) where active caspase-8 is sufficient after downstream caspase-3 activation to lead to execution of apoptosis, in type II cells (RII) which represent the majority of targetable malignancies, the amount of caspase-3 activated via caspase-8 is insufficient to trigger apoptosis. In these Type II cells, death receptor–mediated apoptosis requires amplification of the death signal via activation of the intrinsic (mitochondrial) cell death pathway (aka, mitochondrial apoptosis), in contrast to Type I cells that can rely solely on the extrinsic pathway involving cell surface receptors (RI, RII, RIII).

But what's important to note is that the mitochondrial cell death pathway is tightly regulated by Bcl-2 family proteins (both anti- and proapoptotic), and that although it is also known that overexpression of Bcl-2, Bcl-xL, or Mcl-1 proteins inhibits TRAIL-induced apoptosis, TRAIL-induced cell death depends entirely on the single proapoptotic Bcl-2 family member, Bax (often lost secondary to epigenetic inactivation, or genetic mutations), and therefore it is Bax deficiency that confers resistance against TRAIL-induced apoptosis [6,7].

The key clinically important question to answer is, therefore, what determines the Bax dependency of TRAIL-induced apoptosis? And we now have a decisive answer from the ingenuous research of the German team of Peter Daniel and colleagues [8], namely that it is the endogenous Bak inhibitor Mcl-1 (not Bcl-xL as formerly suspected), making Mcl-1 the major target for sensitization of Bax-deficient tumors for DR–induced apoptosis (over the Bak pathway). And there are confirmatory predictions here: small molecules like the kinase inhibitor sorafenib known to down-regulate Mcl-1 appears to overcome TRAIL resistance, blocking NF-κB, and thereby increasing TRAIL-induced apoptosis in multiple neoplasias (see for example the just published VCU (Virginia Commonwealth University) study of the TRAIL-synergistic antitumor activity of sorafenib plus HDAC inhibition [9] via, in part, reduction in expression of MCL-1; and similarly for roscovitine (Seliciclib), another down-regulator of Mcl-1).

These findings collectively show that resistance against TRAIL consequent to Bax deficiency can be overcome by down-regulation of Mcl-1, which appears to be the common feature in TRAIL/death receptor–mediated apoptosis, and helps organize, integrate and explain multiple diverse mechanisms, and is predictive of the TRAIL-sensitizing role of both the Proteasome inhibitors and the HDAC/DNMT inhibitors (via silencing of pro-apoptotic genes and up-regulating anti-apoptotic genes [14]). This follows the time-honored strategy of deploying mechanisms that overcome resistance processes which are known to be enabled by simultaneous activation of anti-apoptotic and down-regulation of pro-apoptotic mechanisms (such as up-regulation of c-FLIP with down-regulation of caspase-8 or DR4 or DR5).

TRAIL-BASED NAURAL INTERVENTIONS: A PEEK FORWARD

I note in closing, it is intriguing to observe that many natural agents can activate proapoptotic Bax and down-regulate Mcl-1, including quercetin, resveratrol, EGCG, DHA, Vitamin D3, methylseleninic acid (MSeA) especially when coupled with curcuminoids, among others (either by stimulating the expression or appearance of death receptors on cell surface or by amplifying the mitochondrial apoptotic pathway), and most especially (Sabinsa-standardized) curcuminoids (curcumin downregulates XIAP, survivin, BCL-2, HIF1a, claspin, c-MYC, and MCL-1, all implicated in TRAIL resistance, while up-regulating Bax to drive curcumin-induced apoptosis [15, and references therein]), and should this be confirmed - as I anticipate - in pending human clinical studies, it introduces a singularly attractive natural and non-toxic mechanism for TRAIL sensitization and resistance suppression.

METHODOLOGY OF THIS REVIEW

A search of the PUBMED, Cochrane Library / Cochrane Register of Controlled Trials, MEDLINE, EMBASE, AMED (Allied and Complimentary Medicine Database), CINAHL (Cumulative Index to Nursing and Allied Health Literature), PsycINFO, ISI Web of Science (WoS), BIOSIS, LILACS (Latin American and Caribbean Health Sciences Literature), ASSIA (Applied Social Sciences Index and Abstracts), SCEH (NHS Evidence Specialist Collection for Ethnicity and Health) and SCIRUS databases was conducted without language or date restrictions, and updated again current as of date of publication, with systematic reviews and meta-analyses extracted separately. Search was expanded in parallel to include just-in-time (JIT) medical feed sources as returned from Terkko (provided by the National Library of Health Sciences - Terkko at the University of Helsinki). A further "broad-spectrum" science search using SCIRUS (410+ million entry database) was then deployed for resources not otherwise included. Unpublished studies were located via contextual search, and relevant dissertations were located via NTLTD (Networked Digital Library of Theses and Dissertations) and OpenThesis. Sources in languages foreign to this reviewer were translated by language translation software.

REFERENCES

1. Chen JJ, Knudsen S, Mazin W, Dahlgaard J, Zhang B. A 71-gene signature of TRAIL sensitivity in cancer cells. Mol Cancer Ther 2012; 11(1):34-44.

2. Galski H, Oved-Gelber T, Simanovsky M, Lazarovici P, Gottesman MM, Nagler A. P-glycoprotein-dependent resistance of cancer cells toward the extrinsic TRAIL apoptosis signaling pathway. Biochem Pharmacol 2013 Sep 1; 86(5):584-96.

3. Cheung V, Bouralexis S, Gillespie MT. PTHrP Overexpression Increases Sensitivity of Breast Cancer Cells to Apo2L/TRAIL. PLoS One 2013; 8(6):e66343.

4. Kanzaki H, Ohtaki A, Merchant FK, Greene MI, Murali R. Mutations in K-Ras linked to levels of osteoprotegerin and sensitivity to TRAIL-induced cell death in pancreatic ductal adenocarcinoma cells. Exp Mol Pathol 2013; 94(2):372-9.

5. Acunzo M, Visone R, Romano G, et al. miR-130a targets MET and induces TRAIL-sensitivity in NSCLC by downregulating miR-221 and 222. Oncogene 2012 Feb 2; 31(5):634-42.

6. LeBlanc H, Lawrence D, Varfolomeev E, et al. Tumor-cell resistance to death receptor--induced apoptosis through mutational inactivation of the proapoptotic Bcl-2 homolog Bax. Nat Med 2002; 8(3):274-81.

7. Theodorakis, P., E. Lomonosova, G. Chinnadurai. Critical requirement of BAX for manifestation of apoptosis induced by multiple stimuli in human epithelial cancer cells. Cancer Res 2002. 62:3373–3376.

8. Gillissen B, Wendt J, Richter A, et al. Endogenous Bak inhibitors Mcl-1 and Bcl-xL: differential impact on TRAIL resistance in Bax-deficient carcinoma. J Cell Biol 2010 Mar 22; 188(6):851-62.

9. Hamed HA, Yamaguchi Y, Fisher PB, Grant S, Dent P. Sorafenib and HDAC inhibitors synergize with TRAIL to kill tumor cells. J Cell Physiol 2013; 228(10):1996-2005.

10. Holland PM. Targeting Apo2L/TRAIL receptors by soluble Apo2L/TRAIL. Cancer Lett 2013 May 28; 332(2):156-62.

11. Shi J, Zheng D, Liu Y, et al. Overexpression of soluble TRAIL induces apoptosis in human lung adenocarcinoma and inhibits growth of tumor xenografts in nude mice. Cancer Res 2005 Mar 1; 65(5):1687-92.

12. Wajant H, Moosmayer D, Wüest T, et al. Differential activation of TRAIL-R1 and -2 by soluble and membrane TRAIL allows selective surface antigen-directed activation of TRAIL-R2 by a soluble TRAIL derivative. Oncogene 2001 Jul 5; 20(30):4101-6.

13. Dokouhaki P, Schuh NW, Joe B, et al. NKG2D regulates production of soluble TRAIL by ex vivo-expanded human γδ T cells. Eur J Immunol 2013 Aug 20.

14. Jazirehi AR, Arle D. Epigenetic regulation of the TRAIL/Apo2L apoptotic pathway by histone deacetylase inhibitors: an attractive approach to bypass melanoma immunotherapy resistance. Am J Clin Exp Immunol 2013; 2(1):55-74.

15. Reuss DE, Mucha J, Hagenlocher C, et al. Sensitivity of malignant peripheral nerve sheath tumor cells to TRAIL is augmented by loss of NF1 through modulation of MYC/MAD and is potentiated by curcumin through induction of ROS. PLoS One 2013; 8(2):e57152.

16. van Dijk M, Halpin-McCormick A, Sessler T, Samali A, Szegezdi E. Resistance to TRAIL in non-transformed cells is due to multiple redundant pathways. Cell Death Dis 2013; 4:e702.

17. Geserick P, Drewniok C, Hupe M, Haas TL, Diessenbacher P, Sprick MR et al. Suppression of cFLIP is sufficient to sensitize human melanoma cells to TRAIL- and CD95L-mediated apoptosis. Oncogene 2008; 27:3211–3220.

18. Vogler M, Walczak H, Stadel D, Haas TL, Genze F, Jovanovic M et al. Targeting XIAP bypasses Bcl-2-mediated resistance to TRAIL and cooperates with TRAIL to suppress pancreatic cancer growth in vitro and in vivo. Cancer Res 2008; 68: 7956–7965.

Eswar:

Thanks, Eswar, and for providing valuable comments in this discussion.

I specialize in curcumin-based therapeutics for advanced/metastatic cancers, so let me note that the facts re curcumin are more nuanced than commonly stated, however: while it may be true that relatively larger does of SOME curcumin formulations may be required for optimal therapeutic benefit, there are two qualifications that must be noted:

(1) this is not in fact a limiting drawback, since an overwhelming amount of robust data shows and establishes an excellent safety profile even when used orally at well over 10 g/day, and dozens of systematic reviews of both safety and pharmacokinetics demonstrated that in fact curcumin exhibits unparalleled safety amongst pharmacological agents, both natural and traditional [1];

(2) given the poor systemic availability of orally administered curcumin, as you correctly observed, there have been successful efforts to increase oral bioavailability by formulating of curcumin with bioavailability enhancers (such as piperine in Sabinsa-certified curcuminoids), nanoparticles (commercially: Theracurmin), liposomes/phytosomes (commercially: Meriva), and lipidated curcumin (commercially: Longvida), along with soon-to-be-available micelles, intranasal thermosensitive poloxamer hydrogels, and even topical formulations, and with many of these significantly lower dosing is viable. For example, phospholipids enhance the bioavailability / absorption and therapeutic efficacy of curcumin, so in a comparison of a commercial curcumin-phospholipid complex formula BCM-95 CG (Biocurcumax) compared to a curcumin-piperine formula (Sabinsa-certified comparable), the relative bioavailability of curcumin-phospholipid complex was 6.3-fold as high. (And note that a liposomal/phytosome curcuminoid preparation, commercially Meriva, that  has enhanced bioavailability (6X AUC) but more critically higher penetrance, and has been found to have  high penetration and accumulation in the liver with AUC and CMAX up to 20 fold above standard curcuminoids). Thus we can capture equi-efficacy with higher traditional curcumin formulations by using lower dose enhanced-bioavailability formulations.

THE SYSTEMIC AVAILABILITY PARADOX

This paradox is seen by the concurrent truth of two claims: (1) that curcumin has poor systemic oral availability, and (2) that curcumin nonetheless demonstrates high efficacy despite such compromised bioavailability (in part, as I will show below, because we are looking at the wrong thing): thus, clinical pilot studies have associated curcumin consumption with regression of premalignant lesions of the stomach, bladder, soft palate, cervix, and skin [2]. Moreover, curcumin appears to exert both hepatoprotective and nephro-(reno)protective, antithrombotic, myocardial infarction protective, hypoglycemic, anti-inflammatory, antirheumatic and anti-cognitive impairment [3,4].

Given that we are forced by pharmacokinetic and pharmacodynamic data to assume that virtually no curcumin effectively reaches the peripheral organs involved after oral administration, we must realize that the beneficial effects observed in such organs are mediated by curcumin metabolites, congeners, or degradation products (especially demethoxy-CUR and bisdemethoxy-CUR, as well as tetrahydro-CUR, and hexahydro- and octahydro-CUR, all major reductive CUR metabolites). Thus, given the poor absorption from the GI tract, any effects in peripheral tissues must be assumed as mediated by degradation products and/or metabolites (see [5] and references therein). Confirmation comes from the University of Michigan RCT [6] which found that curcumin was absorbed after oral dosing at 10g or 12g daily in humans but needed to be detected not as its base form, but as either glucuronide or sulfate conjugates (not mixed conjugates) in plasma. And in the special environment of colonic mucosa, curcuminoids were detectable in all 23 biopsied participants in a just published clinical pilot study [7], with the major conjugate, curcumin glucuronide, being detectable in 29 of 35 biopsies using a 2.35 g daily dose of Sabinsa-certified curcumin, with excellent tolerability and no safety signals.

HIGH-DOSE SAFETY AND TOLERABILITY

And we know that dosage regimens up to 12 g/day have been demonstrated well tolerated in humans [8], the main problem being not of true GI toxicity but rather of achieving doses above 8 g/day due to the bulk of the compound that needs to be ingested [2]. A similar study found that a third of cancer patients given curcumin (up to 2.2 g daily) experienced radiologically stable disease over the course of 3 months or longer, with one patient even exhibiting abatement in severity of primary colorectal cancer as measured by tumor marker levels, suggesting a potential tumor regression [9], all with exceptional tolerability and safety.

And one recent phase II trial examined the maximal tolerated dose (MTD) of curcumin when co-administered in combination with the taxane docetaxel (Taxotere), with some clinical gains over docetaxel administration alone (5 partial responses (PR) and 3 stable disease responses (SD) out of 14 patients) [10]: the MTD of curcumin (note however, of traditional C3 curcumin complex, not of the newer enhanced-bioavailability formulations) was established as 6000mg/day for seven days when given in combination with a standard docetaxel regime(1 h i.v. infusion every 3 w on d 1 for six cycles [3 months]).

As to my own experience, I have, and track, hundreds and hundreds of advanced cancer patients as my consults across all continents who are using curcuminoids as part of an integrative regimen, and I can generally manage successfully any GI distress in the vast majority through use on demand of over-the-counter (OTC) DGL (deglycyrrhizinated licorice), known to be effective against NSAID and COX-2 inhibitory agents (curcumin is essentially equi-potent with celecoxib (Celebrex) in COX-2 inhibition activity); only for a small proportion of patients might I recourse to an H2RA like ranitidine (Zantac), or a PPI like omeprazole (Prilosec), and for many of these patients I can avoid the issue by switching them to a newer enhanced-bioavailability formulation. If reflux (GERD) is prominent (this is rare), an OTC alginic acid preparation (like Gaviscon) is almost invariably sufficient for effective remedy.

REFERENCES

1. Soni D, Salh B. A Neutraceutical by Design: The Clinical Application of Curcumin in Colonic Inflammation and Cancer. Scientifica (Cairo) 2012; 2012:757890.

2. Cheng AL, Hsu CH, Lin JK, et al. Phase I clinical trial of curcumin, a chemopreventive agent, in patients with high-risk or pre-malignant lesions. Anticancer Res. 2001 Jul-Aug;21(4B):2895-900.

3. Anand, P., Kunnumakkara, A. B., Newman, R. A., and Aggarwal, B. B. ( 2007) Bioavailability of curcumin: problems and promises. Mol. Pharmaceut. 4, 807–818.

4. Zhou, H., Beevers, C. S., and Huang, S. ( 2011) Targets of curcumin. Curr. Drug Targets 12, 332–347.

5. Metzler M, Pfeiffer E, Schulz SI, Dempe JS. Curcumin uptake and metabolism. Biofactors 2013 Jan-Feb; 39(1):14-20.

6. Vareed SK, Kakarala M, Ruffin MT, et al. Pharmacokinetics of curcumin conjugate metabolites in healthy human subjects. Cancer Epidemiol Biomarkers Prev 2008; 17(6):1411-7.

7. Irving GR, Howells LM, Sale S, et al. Prolonged biologically active colonic tissue levels of curcumin achieved after oral administration--a clinical pilot study including assessment of patient acceptability. Cancer Prev Res (Phila) 2013; 6(2):119-28.

8. Lao CD, Ruffin MT, Normolle D, et al. Dose escalation of a curcuminoid formulation. BMC Complement Altern Med 2006; 6:10.

9. Sharma RA, McLelland HR, Hill KA, et al. Pharmacodynamic and pharmacokinetic study of oral Curcuma extract in patients with colorectal cancer. Clin Cancer Res 2001; 7(7):1894-900.

10. Bayet-Robert M, Kwiatkowski F, Leheurteur M, Gachon F, Planchat E, Abrial C, Mouret-Reynier MA, Durando X, Barthomeuf C and Chollet P. Phase I dose escalation trial of docetaxel plus curcumin in patients with advanced and metastatic breast cancer. Cancer Biol Ther. 2010; 9:8-14.