We had a similar problem in nude mice with MDA-MB-231 breast cancer cells. Only about 50% of mice developed tumors that grew. We injected again the mice that did not develop tumors, but only a few more (maybe another 10%) developed tumors the second time. We think there was a problem with that batch of mice, maybe their immune system was not suppressed well, since usually almost all the mice develop tumors. As far as I know, U87 also are supposed to have a good take, and 5 million is a lot.

Dear Ela,

Thanks for your kind information

Did you inject the cell into same set of animals previously injected with MDA-MB-231?

Should immune suppression with cyclophoasphamide will work for reinjection of cell in previously injected animals ?

Yes, we injected a second time into animals that did not develop tumors after the first injection (1 million cells/mouse). Only a few of them developed tumors after the second injection. 

Using cyclophosphamide doesn't make sense to me, since it is also used to treat cancer.... And anyway, that would be a completely different mouse model and you won't be able to compare it to regular nude mice.

Dear Anuj,

if the first time the injection of tumor don't work it's possible that or that mice weren't completely immunodeficient or that you have some problems with the cells for example micoplasma contamination. Unlikely when mice are injected with tumor cell lines  the engrafment is never 100%, it depends by cell line and by health status of mice. I advise against the reinoculation of mice, better to understand the motivation of regression.

bye

Dear Antonio

Thanks a lot for your valuable information !!!!!