Try sodium-ethoxide. Prior to IHC, you must retrieve epitopes. I suggest heat-induced retrieval or citrate buffer method (the question is whether you used formaldehyde for fixation). And sometimes, depending on antigens wanted to be disclosed, there is no need to deplasticize at all (as sodium-ethoxide can destroy some of the epitopes).

Dear Michal, thanks for your helpful recommendation!  I've just purchased sodium-ethoxide, which I will try it on.  Can you give me more details about it?  Do I have to dilute it (with what)? How long do I have to submerge the spurr embedded slide into the sodium-ethoxide solution?..., etc.  I've performed MMA embedded slide successfully, which fixed with NBF.  I deplasticize them with Xylene, performed heat-induced antigen retrieval with citrate buffer, then applied with antibody...  Again, thanks for the help and thanks again in advance for additional details.

Dear Xiaojian,

I am happy being helpful! I would try 1:1 saturated S-E with 100% ethanol, exposed for 10-40 min. And rehydration as a next step.

I am sending two papers where, I expect, you will find all your answers and even established procedures for deplasticizing.

https://www.ncbi.nlm.nih.gov/pubmed/10936453

http://journals.sagepub.com/doi/pdf/10.1177/002215549704500117

Dear Michal,

I really appreciate your wonderful help for the details and references!

Best wishes,

Xiaojian

Honestly adding (an)other possibility(-ies) (cf. attached recipe[s],articles.... 

best wishes, good luck and regards, WM

PS: Edited 2017-04-06,19:12 MEST for the following supplementary information on jpg. Fig. /Table 5-5-:    

a) if you save the jpg you will find some hint on the original source of the table.  I once in my "former, professional" life copied the table and added it to my works collection of important working SOP'S and protocols.      

As of today I found a reference which might contain the table 5-5 on page 100:  cf: http://onlinelibrary.wiley.com/doi/10.1002/pi.4980190342/full    

BOOK review (1987)  by C.A. WHITE and J.F. KENNEDY,  for:

Immuno enzyme techniques in cytochemistry: W. D. Kuhlmann, Verlag Chemie, Weinheim, 1984. pp. xiii + 162  

b) since I am not quite sure about the / that original source, I would like to point also to a web-file (by the same author)  I found most recently: 

Immuno-staining with resin embedded tissue

WOLF D. KUHLMANN, M.D. Division of Radiooncology,

Deutsches Krebsforschungszentrum, 69120 Heidelberg, Germany                                © Prof. Dr. Wolf D. Kuhlmann, Heidelberg                  10.09.2006    which can be found @ http://www.immunologie-labor.com/cellmarker_files/IET_protocols_02.pdf

Dear Dr. Muss, 

I really appreciate your help. Your recommendation was instrumental to our success in developing the spurrs deplasticization method for IHC.

Best regard,

Xiaojian

Dear Dr. Xiaojian Li, you're welcome ! Best wishes, & good luck, WM

PS:  it might also be interesting at this point (for convenience of other readers too) to mention the following "old" thread (which I was able to find in my e-data files on the matter right now):  https://www.researchgate.net/post/Does_anyone_have_experience_in_extracting_DNA_from_plastic-embedded_tissue? 

EDIT:

Due to the fact that the former LINK to https://www.researchgate.net .... does (DIDN'T) not work this time [2019-11-18, 14:10 UTC(+1)] I try to put another URL here (copy and paste from the original thread):

https://www.researchgate.net/post/Does_anyone_have_experience_in_extracting_DNA_from_plastic-embedded_tissue