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For as long as I can remember, there has been lurking excitement regarding the possibility that our immune systems can find and destroy cancer cells. The history of well-documented spontaneous remissions goes back decades and is briefly reviewed here. I have personally never seen a spontaneous remission of cancer, although I have had patients who have done far better than anyone would have expected, suggesting that something must have slowed down their tumor progression.
In prostate cancer, one of the early hints that it might be possible to stimulate an immune attack on the disease came from the studies on Provenge (Sipuleucel-T). My colleagues and I placed several patients on the trials that led to approval of this “vaccine” by the FDA. These studies have continued to demonstrate improved survival of patients with metastatic disease who have failed hormone therapy, although the trials were all done before the availability of the newer ADT drugs abiraterone, enzalutamide, and apalutamide. On the other hand, in spite of the optimistic data we obtained in another vaccine trial on a product known as prostvac, the pivotal trial to prove efficacy failed. It is possible that the vaccine produced modest efficacy, but the signal was drowned out by treatment with the new ADT agents.
As anyone who watches the evening news or other TV-ad-saturated programs aimed at us seniors, other cancers – especially melanoma, lung, bladder, kidney and a few additional ones have been more “easily” treated with newer immune therapies known as check point inhibitors. The idea here is that our normal immune system has built in “braking systems”, the best studied and clinically utilized to date being the PD-1/PDL-1 mechanism. If we immunize you against, for example, measles – you want a vigorous immune response, but you don’t want your entire immune system to keep working on fighting measles. There are other threats it needs to be on guard against. Shutting down the T-cells that fight viruses and cancer involves the Programed Death receptor-1 on these T-cells with a specific protein, Programed Death receptor Ligand-1. Cancer cells can take advantage of initiating this same braking system by releasing their own PDL-1 that will kill the incoming tumor-fighting T-cell. This devious cancer mechanism to avoid our immune systems can be blocked by therapeutic antibodies directed against either the receptor or the PDL-1 ligand protein.
At the recent ASCO meeting, it was revealed that selected metastatic lung cancer patients who have an activated PD-1/PDL-1 braking system are now more effectively treated with pembrolizumab (Keytruda) than chemotherapy. It is emerging that the subgroup of patients who have tumors that are genetically highly unstable, (regardless of tumor type) with lots of mutations leading to abnormal proteins that can stimulate an immune response, may all benefit from PD-1/PDL-1 directed therapy. These patients, including prostate cancer patients can be identified by testing their tumors for microsatellite instability or mismatch repair deficiency. At a practical level, however, when and how to test prostate cancers for such biomarkers remains challenging. Last week at the ASCO annual meeting, Dr. De Bono from the UK reported results on treating patients with metastatic prostate cancer who had progressed on hormones and chemotherapy (docetaxel) with pembrolizumab. 17/163 patients had ≥30% shrinkage of their tumors, but overall results were disappointing with only 11% of patients having ≥50% decline in PSA. Testing for the presence of PDL-1 was not particularly predictive of which patient would benefit most. However, this way of treating prostate cancer will eventually lead to important progress in my opinion. Combining vaccines with the checkpoint inhibitors is currently being studied, and there are other checkpoint drugs and targets that are in development as well. Timing the checkpoint drugs with hormonal therapy or radiation therapy may also find optimal ways of stimulating an immune response. The field of immuno-oncology is an exciting new frontier and well worth keeping your eyes on.