Well, it is vital to establsi hthe IC50 value of a chemical or compound to prove it as toxic.

You should not go ahead before you do not work on the IC50 value of said compound. You may use any in vitro method for assessing the same.

It's not precised why you were unable to determine IC50: was the tested compound too toxic? (then simply dilute it properly), or it was not toxic in the tested range of concentrations? In the latter case the problem can be that the  toxic concentrations exceed the solubility of a compond in the chosen system. If you can determine some toxicity (but can't achieve IC50 figures) you may use mathematical extrapolation (for instance - linear regression model) to calculate IC50.

What kind of chemical or nanomaterial did you test? Check the cells by light microscope (after MTT incubation (before adding of DMSO)).

Thanks All for your contributions. Actually the compound shows some cytotoxicity, but i could not determine the ic50 from concentration/cell viability curve.

@Rameshwar what do you mean by other methods of assessing same? could you please elaborate.

You can use hepatic cell line for assessing the IC50 value of the chemical. Also you can perform MTT assay.

Ok, thanks

In my opinion if you are not achieving IC50 but you have a styrong rationale for your model and for the treatment doses you don't have to find IC50 at any cost beacuse your data are scientifically based. For example if you treat your cells with concentrations equal to environmental levels or to human-related doses you don't have to perform additional assays... What is the extent of the effects you have recorded???

I think that you could focalize on other sublethal endpoint (genotoxicity, ROS production, inflammation) to verify if other mode of action and biological responses could be induced by your treatment.

You could also try to exatrpolate IC50 using some powerful statistical programs.

I am using Cancer Cells

Why couldn't you find it? It's not possible to determine whether this is due to the system or the experiment execution without more details. Did you get a standard curve?

How can you know that your compound is "cytotoxic" while you did not record data about it.

Furthermore, small comments ...

The IC50 / GI50-related values obtained by means of a colorimetric assay do actually not translate “cytotoxic” effects.

Cytotoxicity means "cell killing effects".

A colorimetric assay can only bring "relative global growth inhibition information" because it is a relative test in which you compare the ODs of a treated cell population to the ODs of a control condition (untreated cells) arbitrarily scaled at 100%.

When you obtain a concentration (for a given compound) decreasing by 50% the global growth (after x days), i.e. the GI50 concentration (or the IC50 as commonly used in the literature) you do not know whether your compound of interest killed 50% of the cells (cytotoxic effects), whether it inhibited 50% of the cell proliferation (cytostatic effects), whether it detached 50% of the cells (anti-adhesive, i.e. "in vitro antimetastatic" effects), etc..., etc...

Once you have determined the GI50 / IC50 concentration for a given compound on a given cell line, you must use complementary biochemical and/or morphological techniques to determine whether your compound is cytotoxic, cytostatic, anti-adhesive, etc..., etc...

The two attached articles by Galluzzi and colleagues (2012, 2015; Appendix-1 and Appendix-2) are of great help in this domain.

The attached article by Kornienko et al. (2013; Appendix-3) reviewed various chemicals that are able to induce non-apoptotic cell deaths in cancer cells.

Coming back to the IC50 / GI50 values obtained by means of a colorimetric assay (as for example the MTT one):

in the Mathieu et al. (2009 (Appendix-4) and 2015 (Appendix-5)) articles, the MTT test-related GI50 concentrations relate to actual cytotoxic effects.

In the Lefranc – Nunzo et al. (2013; Appendix-6) article, the MTT test-related GI50 concentrations relate to cytotoxic effects that in turn do not relate to apoptosis … This means that each cytotoxic effect does not “universally” translate into pro-apoptotic ones.

In the Van Goietsenoven et al. (2010; Appendix-7) article, the MTT test-related GI50 concentrations relate to cytostatic effects, neither to cytotoxic nor to pro-apoptotic ones.

Be aware that you cannot always translate the MTT test-related growth inhibition of a given compound into a precise GI50 value. Some compounds reach a “plateau” of inhibition (see Lefranc – Nunzo et al., 2013; Appendix-6).

Lastly, you can also have "false" data generated with the MTT colorimetric assay (see the attached article by Chan et al. (2013; Appendix-8) and the first NCI-60-cell line-related article (Alley et al., 1988 (Appendix-9)).

The US NCI set up a fantastic tool to characterize the effects of a given drug in terms of growth inhibition in a panel of 60 cancer cell lines belonging to >10 histopathological types (Alley et al., 1988; Appendix-9).

The US NCI clearly defined by means of the combination of the GI50 (growth inhibition), the LD50 (lethal dose by 50%) and the TGI (total growth inhibition) how to make the difference between a cytotoxic and a cytostatic compound:

The US NCI-related GI50 value corresponds to a global growth decrease by 50% induced by a compound on a given cell line “x” days after having cultured the cells with the drug and in comparison to an untreated control condition (= 100%) grown during the same time;

The US NCI-related LD50 value corresponds to the a global growth decrease by 50% induced by a compound on a given cell line “x” days after having cultured the cells with the drug and in comparison to the initial number of cells in the untreated control condition;

The TGI is the US NCI-related parameter to determine the concentration needed to kill 100% of the treated cells.

It is by comparing the GI50 to the LD50 value that one can determine whether a compound is cytotoxic or cytostatic, and not at all with the sole GI50 value.

We are using morphological approaches in my research unit to determine whether a compound is cytotoxic or cytostatic (see Lefranc-Nunzo et al., 2013; Mathieu et al. 2009 (Appendix-4), 2015 (Appendix-5); Van Goeitsenoven et al., 2010 (Appendix-7)).

The US NCI is not so far for having tested about one million of anticancer drugs, whose data are publicly available on the NCI website https://dtp.cancer.gov/databases_tools/data_search.htm

I actually benefited several times from the amazing help of the NCI in identifying the mechanism of action of an innovative anticancer compound (see for example Frederick et al. JMC 2011 (Appendix-10)).

Hoping that this long explanation would not be too boring,

Best regards

Robert