Drug Targeting Tumor Suppressor Shows Promise in First Human StudyAn experimental drug that reactivates mutant forms of the tumor suppressor protein p53 is safe for humans, according to results from a phase I trial. The drug, APR-246, also stimulated signaling pathways that control p53 in tumor cells isolated from peripheral blood.
The study, published in the Journal of Clinical Oncology, was led by Dr. Sören Lehmann of Karolinska University Hospital in Stockholm.
The findings represent “a major step forward in targeting the most frequently altered pathway in cancer,” wrote Drs. Brian D. Lehmann and Jennifer A. Pietenpol of Vanderbilt University School of Medicine in an accompanying article.
The p53 protein suppresses tumor growth by increasing the expression of genes that slow the cell cycle, that prevent cells from dividing, or that cause programmed cell death (apoptosis). At least half of all tumors develop inactivating mutations in TP53, the gene that produces p53, allowing the tumor to evade this regulation. APR-246 counteracts TP53 mutations by restoring the gene-regulatory activity of mutant p53 protein and by inducing the death of cancer cells.
The 22 participants enrolled in the trial had various forms of leukemia, hormone-refractory metastatic prostate cancer, non-Hodgkin lymphoma, or multiple myeloma. These patients were included because prostate cancers have a high rate of TP53 mutations and because, in preclinical studies, leukemia cells were particularly sensitive to drugs that target p53.
The patients received intravenous infusions of APR-246 for 4 consecutive days. After a follow-up period of 17 days, the most common side effects were fatigue, dizziness, headache, and confusion.
While the trial was not designed to assess the drug’s antitumor effects, several patients showed clinical responses. To investigate the biologic activity of APR-246, Dr. Lehmann and his colleagues analyzed circulating tumor cells obtained before and after the 4-day treatment from the six patients who had these cells. Four of these patients had fewer proliferating cells after receiving APR-246. The investigators also observed signs of apoptosis and increased expression of several p53 target genes.
Microarray analysis of RNA isolated from the circulating tumor cells showed changes in genes responsible for regulating cell growth and death. The level of expression of genes that promote apoptosis appeared to correlate with the dose of APR-246.
Trials are being designed to investigate the effects of higher exposures to APR-246 through longer infusion times and of combining APR-246 with other chemotherapy drugs, most of which depend on a functional version of p53 to be effective.