Study of Docetaxel +153 Sm-EDTMP in Patients With Hormone-Refractory Prostate Cancer

Prostate cancer remains the leading cancer diagnosed in men in the USA. It is estimated that 232,090 new cases will be diagnosed in 2003, which account for about 30% of all cancer cases diagnosed in men.(1) Prostate cancer is still the second leading cause of cancer death in men (after lung cancer) in the US with estimated 28,900 men dying of prostate cancer in 2003. Although 79% of men will present with early stage disease (i.e., local or regional) at the time of diagnosis, a substantial number of men will present with metastatic disease. (2) Unfortunately many of the patients who present with local disease and are treated with local therapy will eventually experience disease recurrence. Initial treatment for patients with prostate cancer in advanced stages has been through androgen deprivation. Approximately 85% of men will have an objective response to hormonal therapy initially but eventually these patients will develop progressive and eventually fatal disease. Treatment options for patients with HRPC include systemic cytotoxic chemotherapy and /or palliation of symptoms (especially pain) (3). Single agent and combination chemotherapy regimens have been associated objective response rates ranging from 40-70% in patients with HRPC (4). No survival advantage has been shown with any treatment in phase III studies. More effective systemic therapies are needed if we are to have an impact on the morbidity and mortality caused by the disease. (5)

A newer bone targeted approach is the combination of radioisotopes with chemotherapy. In addition to its inherent systemic benefits chemotherapeutic agents can also act as radiosensitizers when combined with radiopharmaceutical. Preliminary clinical experience with the combination of radiopharmaceuticals and systemic chemotherapy indicate that this approach is feasible tolerable and potentially effective. In a randomized phase II trial metastatic HRPC patients were treated with chemotherapy: Ketoconazole and Doxorubicin alternating with Vinblastine and Estramustine (KAVE regimen). Seventy-two patients who were stable or responded after 2-3 cycles were randomized to receive a consolidation treatment with additional 6 weekly doxorubicin treatments with a single dose of 89Sr or weekly doxorubicin alone. Patients receiving the combined consolidation treatment had a significantly longer time to progression and survival (27.7 vs.16.8 months. P=0.0014).(20)

Docetaxel clearly represents the most active single agent for the treatment of metastatic prostate cancer. The radiosensitizing properties of Docetaxel are well documented in preclinical and clinical experiments. The most likely underlying mechanism is a G2M block in the cell cycle induced by docetaxel and this will result in cycling cells at the most radiosensitive phase of the cell cycle.

Besides a palliative role, current clinical and preclinical data suggest that a bone targeted approach may represent a potentially useful component of the overall therapeutic approach for metastatic prostate cancer. Randomized, double-blind, placebo controlled trials suggest that bisphosphonates and endothelin A antagonists may delay the progression of prostate cancer in bone. These clinical observations of relatively large trials have provided impetus for the design of large definitive studies to evaluate the role of various bone targeted approaches for the treatment of prostate cancer.

The currently proposed trial represents a preliminary evaluation of the safety of the docetaxel +153 Sm combination. This study may also provide important preliminary efficacy information with this combination.

The combination regimen was designed to explore clinically the possible interactions between docetaxel and samarium-153-EDTMP. The block in the G2/M phase following the administration of docetaxel results in the accumulation of cells in the most radiosensitive phase of the cell cycle. Pre-clinical data suggest that this is likely to occur 24-48hrs post administration of docetaxel. This is followed 24 hours later by the administration of the radiopharmaceutical which has a short physical half life of 46.3 hours. The bone uptake of the complexed sm153- EDTMP molecule will be effectively taken up at the bone metastatic site within this time interval. To target cells repopulating the cell cycle following the initial block+ cell kill in the mitotic phase, we planned a Q3 week x 2 administration of docetaxel on Day 1 and Day 22. Following a 8 week of rest (weeks 5 -12), treatment is repeated for a maximum of 2 cycles or until the development of dose-limiting toxicity or disease progression.

Samarium is given in 12 week intervals. Docetaxel is a radiosensitizer that has shown activity in prostate cancer given to patients on both Q3 week and Q week schedules.

Based on a personal communication from Dr. Theodore DeWeese's (Johns Hopkins Hospital Radiation Oncologist) preclinical data suggests that 24 hours after a dose of docetaxel, there is a maximum G2M arrest. This represents the rationale for the bolus dose of docetaxel for the combined schedule.