Researchers lead by Prof. Daniel Petrylakwill reports that, a new drug that specifically targets a protein found on the surface of prostate cancer cells has performed well in a phase I clinical trial, and a phase II trial has started. The drug reduced levels of circulating tumour cells (CTC) and levels of prostate specific antigen (PSA), a marker for tumour activity, in patients who had already failed previous chemotherapy and hormone treatments.
The drug is made up of a monoclonal antibody, which targets a protein called prostate specific membrane antigen (PSMA), linked to a cancer cell-killing drug called monomethyl auristatin E (MMAE see structure), which disrupts tubulins, the tiny molecules inside a cell that are essential for cell division. The PSMA antibody drug conjugate (PSMA ADC) binds to the PSMA on the surface of the prostate cancer cell and is absorbed into the cell where the MMAE is released, causing cell cycle arrest and cell death.
Prof.Daniel Petrylak, who was Professor of Medicine at Columbia University Medical Center (USA) when the phase I trial started and who is now Director of the Prostate Cancer Program/Genitourinary Cancer Program and co-director of the Signal Transduction Program at Yale University Medical Centre (USA), said: "By conjugating the antibody with a chemotherapeutic agent, we hoped that this would lead to more targeted therapy, which would have fewer toxic side-effects and would be more effective against the cancer."
Prof Petrylak and his colleagues from other US
The researchers detected antitumour activity among the patients who were treated at the higher doses. About half of the patients who received doses of 1.8 mg/kg or more showed either a 50% or more reduction in PSA levels, or a fall in CTC in the blood to less than five cells per 7.5 ml of blood, or both.
The drug was generally well tolerated by patients, although levels of white blood cells were significantly reduced (neutropenia) at the highest dose of 2.8 mg/kg and one patient died. The researchers say the cause of the death is unclear.
Prof Petrylak said: "These results show that PSMA ADC has anti-tumour activity in patients who have failed up to two prior chemotherapies and hormone therapy. We have initiated a phase II trial of up to 75 patients in which the recommended dose will be 2.5 mg/kg. This new trial will evaluate responses in PSA and CTC; it will evaluate control of metastases in bone, internal organs and lymph nodes; and it will look at the effect on pain. Safety also will be assessed.
"The fact that this new targeted therapy is active against the most advanced forms of prostate cancer is encouraging, as few or no therapeutic options are available at present."
