A new therapy for metastatic prostate cancer has shown considerable promise in early clinical trials involving patients whose disease has become resistant to current drugs.
Chemists and biologists at UCLA and colleagues at several other institutions, including Memorial Sloan-Kettering Cancer Center, have created a new drug to treat a particularly lethal form of the disease, known as castration-resistant prostate cancer, or CRPC. Also referred to as hormone-refractory prostate cancer, CRPC is resistant to further treatment by anti-hormone drugs such as Casodex and Eulexin.
In an article published April 9 in the advanced online edition of the journal Science, the scientists describe the development and testing of two novel compounds, MDV3100 and RD162, which block the androgen receptor (AR) in CRPC cells, and report results from clinical trials in which MDV3100 was found to lower prostate-specific antigen (PSA) levels — a marker for tumor growth — in men with CRPC.
The new, small organic molecule MDV3100 was "designed as a very strong antagonist of the androgen receptor to stop the growth of any prostate cancer that requires the AR for propagation, which includes most forms of prostate cancer," said Michael Jung, UCLA professor of chemistry and biochemistry and a researcher at UCLA's Jonsson Comprehensive Cancer Center, whose research group synthesized both MDV3100 and RD162.
The biology research was carried out in the UCLA departments of medicine, urology and pharmacology by Charles Sawyers and his research group; Sawyers has since moved to Memorial Sloan-Kettering Cancer Center in New York, where he serves as chair of the human oncology and pathogenesis program. The UCLA patents for both compounds were licensed by the pharmaceutical company Medivation Inc., which chose to test MDV3100 in clinical trials.
The drug has successfully completed Phase 1 and Phase 2 clinical trials, and the Food and Drug Administration has agreed to allow Medivation to begin what Jung described as "the pivotal Phase 3 clinical trials."
The results of clinical studies with MDV3100 were described at the 2009 ASCO Genitourinary Cancer Symposium in February by the trials' principal investigator, Dr. Howard Scher of Memorial Sloan-Kettering Cancer Center. In general, the drug, at 240 mg once a day, was very effective at lowering PSA levels and also in reducing the number of circulating tumor cells, without any significant toxicity.
"I think it is quite likely that the exciting results seen in the smaller population will also be evident in the larger Phase 3 trial and that the drug could be approved for use in the next few years," said Jung, who is also a member of the California NanoSystems Institute (CNSI) at UCLA.
Of 30 men with anti-androgen–resistant prostate cancer who received low doses of MDV3100 in the multisite Phase 1/2 trial designed to evaluate safety, 22 showed a sustained decline in PSA levels, an indication that their cancer was responding favorably to the drug. This trial is still underway, and results from a total of 140 patients receiving higher doses of the drug will be reported within the next year, Sawyer said.
The Phase 3 clinical trial will evaluate the drug's effect on survival in a large group of patients with metastatic prostate cancer.
MDV3100 and RD162 are second-generation anti-androgen therapies that prevent male hormones from stimulating the growth of prostate cancer cells. These new compounds appear to work well even in prostate cells that have a heightened sensitivity to hormones; that heightened sensitivity makes prostate cancer cells resistant to existing anti-androgen therapies.
Approximately 186,000 new cases of prostate cancer are diagnosed each year in the United States. The male hormones testosterone and dihydrotestosterone, which are also known as androgens, spur the growth of prostate cells, and drugs that block the receptors for these hormones are the most common treatment for the disease in its advanced, metastatic stage. Anti-androgen drugs, such as bicalutamide (Casodex), suppress the growth of cancer cells temporarily, but in most patients, the cancer ultimately develops resistance to drugs. Approximately 29,000 men in the United States die each year from the disease.
Prostate cancer becomes resistant to anti-androgen drugs when cancer cells begin to increase production of the androgen receptor, Sawyers said. When the level of androgen receptors on the cells' surface reaches a certain level, the drugs that originally suppressed the cancer actually begin to stimulate cancer growth.
Because of this backlash effect, many scientists have questioned whether blocking the androgen receptor is a wise course of action. Sawyers and his colleagues, however, believe that blocking the receptor is critical to successful treatment. They set out to design a new generation of drugs that can block the androgen receptor without unwanted side effects, even when levels of the receptor are high.
Researchers in Jung's and Sawyers' laboratories based their designs on a drug that tightly attaches to the site on the androgen receptor that binds with testosterone. If that site is blocked, the hormone cannot bind to prostate cells and tell the receptor to stimulate growth. Using this initial drug as a chemical scaffold, the researchers synthesized nearly 200 slightly different versions of the drug. They tested each one in the laboratory on prostate cancer cells that had been engineered to produce high levels of androgen receptor.
This screening yielded MDV3100 and RD162, molecules which tightly bind to the androgen receptor and do not show the cancer-stimulating effect of bicalutamide and other current anti-androgen drugs. The molecules were good candidates for drugs, because they are readily absorbed into the blood when taken orally and they persist in the bloodstream. The researchers tested the new drugs' effectiveness in mice with tumors derived from drug-resistant prostate cancer cells.
"To our delight, we found that these compounds caused very dramatic shrinkage of tumors in the mice," Sawyers said. "While treating these animals with bicalutamide produced a modest effect on their tumors, the new drugs caused the tumors to shrink dramatically, and in some animals almost completely."
Sawyers said the new drugs bind tightly enough to the natural hormone-binding site on androgen receptors to prevent most of them from functioning, even in cells with many androgen receptors.
The promising laboratory studies led Medivation to license the drugs for commercial development.
Medivation has received permission from the FDA for a large Phase 3 clinical trial of MDV3100 on about 1,200 patients with anti-androgen-resistant disease. This study will assess MDV3100's effect on cancer survival and will take several years.
While the preliminary results are promising, Sawyers said his laboratory will continue to seek further improvements in drug therapy for prostate cancer.
"There were some men in the initial trial in which the drug didn't work at all, and we want to find out why," he said. "It may be because the drug is not potent enough to overcome resistance due to androgen receptor over-expression. Or it may be that the cancers in these men are not driven by the androgen receptor anymore. Also, there were men who initially received benefit from the drug but then relapsed, and their PSA levels came back up. We want to understand the mechanism of that relapse and to try to develop drugs that prevent that renewed resistance."
For years, no treatment was available for CRPC; recently paclitaxel — a strongly cytotoxic drug — was approved.
In addition to Sawyers' and Jung's teams, researchers from the Oregon Health and Science University, the University of Washington and Medivation contributed to the research.
This research was supported by the National Institutes of Health, the Department of Defense, the Prostate Cancer Foundation and Medivation and was conducted through the Prostate Cancer Clinical Trials Consortium.
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