Androgen priming and cytotoxic chemotherapy in advanced prostatic cancer

The testosterone paradox of advanced prostate cancer: mechanistic insights and clinical implications

The discovery of the benefits of castration for prostate cancer treatment in 1941 led to androgen deprivation therapy, which remains a mainstay of the treatment of men with advanced prostate cancer. However, as early as this original publication, the inevitable development of castration-resistant prostate cancer was recognized. Resistance first manifests as a sustained rise in the androgen-responsive gene, PSA, consistent with reactivation of the androgen receptor axis. Evaluation of clinical specimens demonstrates that castration-resistant prostate cancer cells remain addicted to androgen signalling and adapt to chronic low-testosterone states. Paradoxically, results of several studies have suggested that treatment with supraphysiological levels of testosterone can retard prostate cancer growth. Insights from these studies have been used to investigate administration of supraphysiological testosterone to patients with prostate cancer for clinical benefits, a strategy that is termed bipolar androgen therapy (BAT). BAT involves rapid cycling from supraphysiological back to near-castration testosterone levels over a 4-week cycle. Understanding how BAT works at the molecular and cellular levels might help to rationalize combining BAT with other agents to achieve increased efficacy and tumour responses.

The Role of Testosterone in the Treatment of Castration-Resistant Prostate Cancer

Most men with metastatic prostate cancer who are treated with androgen deprivation therapy will eventually develop castration-resistant disease. In this review, we examine the molecular mechanisms that constitute castration resistance and how these processes may be exploited using testosterone-based therapies. We detail how the utilization of superphysiologic doses of testosterone at regular intervals, followed by a rapid clearance of testosterone through continued chemical castration, also known as bipolar androgen therapy, offers an especially promising therapeutic approach. We investigate the historical basis for this modality, detail recent early-phase clinical trials that have demonstrated the feasibility and efficacy of this treatment, and describe an ongoing clinical trial comparing this modality to a currently accepted standard of care, enzalutamide, for castration-resistant prostate cancer. Finally, we explore how this treatment modality will continue to be refined in the future.

Phase 1 trial of high-dose exogenous testosterone in patients with castration-resistant metastatic prostate cancer

BACKGROUND

Growth of selected castration-resistant prostate cancer (CRPC) cell lines and animal models can be repressed by reexposure to androgens. Low doses of androgens, however, can stimulate tumor growth.

OBJECTIVE

We performed a phase 1 clinical trial to determine the safety of high-dose exogenous testosterone in patients with castration-resistant metastatic prostate cancer (CRMPC).

DESIGN, SETTING, AND PARTICIPANTS

Patients with progressive CRMPC who had been castrate for at least 1 yr received three times the standard replacement dose of transdermal testosterone.

INTERVENTION

Cohorts of 3-6 patients received testosterone for 1 wk, 1 mo, or until disease progression.

MEASUREMENTS

Toxicities, androgen levels, prostate-specific antigen (PSA) assays, computed tomography (CT) scans, bone scintigraphy, positron emission tomography (PET) scans, and metastatic tumor biopsy androgen receptor levels were assessed.

RESULTS AND LIMITATIONS

Twelve patients were treated-three in cohorts 1 and 2 and six in cohort 3. No pain flares were noted. One patient came off study because of epidural disease, which was treated with radiation. Average testosterone levels were within normal limits, although dihydrotestosterone (DHT) levels on average were supraphysiologic in cohort 3. One patient achieved a PSA decline of >50% from baseline. No objective responses were seen. For cohort 3, median time on treatment was 84 d (range: 23-247 d).

CONCLUSIONS

We have demonstrated that patients with CRMPC can be safely treated in clinical trials using high-dose exogenous testosterone. Patients did not, on average, achieve sustained supraphysiologic serum testosterone levels. Future studies should employ strategies to maximize testosterone serum levels, use contemporary methods of identifying patients with androgen receptor overexpression, and utilize PSA Working Group II Consensus Criteria clinical trial end points.

TRIAL REGISTRATION

ClinicalTrials.gov; NCT00006044.

A pilot trial of chemohormonal therapy for metastatic prostate carcinoma

Fifteen patients with previously untreated metastatic prostate cancer were treated on a pilot trial with a combination of maximal androgen blockade plus intermittent cytotoxic therapy after androgen priming to stimulate cell division. Androgen blockage was carried out using a gonadotropin-releasing hormone analog (leuprolide) plus a nonsteroidal antiandrogen (flutamide). Carboplatin (CBDCA) (800 mg/m2) was given intravenously every 28 days, preceded for 3 days and followed for 3 days by androgen treatment with fluoxymesterone (5 mg orally twice a day), during which time flutamide was discontinued. Three patients (20%) achieved a complete response (CR), and eight patients (53.3%) achieved a partial response (PR). Four patients (26.7%) had stable disease (SD). The median progression-free survival (PFS) time was 31 months. Nine of 15 patients (60%) remain alive with a median follow-up time of 42+ months (range, 22 to 54 months). Grade 4 thrombocytopenia and Grades 3 or 4 leukopenia were experienced in 87% and 80% of patients, respectively, requiring dose reductions of CBDCA in 85% of the cycles. Six of 15 patients experienced a flare in bone pain with androgen priming. There were no associated spinal cord compressions; however, exclusion of impending spinal cord compression was required before entrance on study.

Carcinoma of the prostate: pathology, staging, and treatment

Prostate cancer is the most common malignancy affecting American men over the age of 50. Its incidence increases with each decade. It is usually discovered by rectal examination performed during physical examination or incidentally found on histologic sections of a prostate removed to alleviate urinary obstructive symptoms. About 50% of patients have clinically localized disease at presentation. The majority of prostate cancers grow slowly, metastasize late, and are not the primary cause of death. Before recommending treatment to a patient with prostate cancer, the treating physician should assess the extent of malignancy and determine which therapy, if any, would favorably influence the course of disease with the least influence on the general quality of life. Patients with disease limited to the prostate are offered curative therapy with either radical prostatectomy or radiation therapy. Those with locally extensive disease are treated with external beam irradiation or with androgen ablation. Metastatic disease is treated by androgen ablation. Chemotherapy is reserved for those patients who fail hormonal treatment.

Estramustine phosphate (estracyt) following androgens in men with refractory stage D2 prostate cancer

Twenty-two orchiectomized men with progressive stage D2 prostate cancer were treated with a 3-week cycle of estramustine phosphate (EMP: from day 3 to day 21) and androgen priming (from day 1 to day 4). A partial response according to the NPCP-USA criteria was shown in 4 of 20 evaluable patients. Median progression-free survival of all patients was 24 weeks (range, 4-48) and median survival, 42 weeks (range, 4-112). Although in two cases treatment had to be stopped due to a marked increase in bone pain, no life-threatening side effects were observed. The androgen sensitivity of tumors was supported by the occurrence of increase in prostatic phosphatase and in bone pain in most patients. In this group of patients, androgen priming did not seem to potentiate the effectiveness of EMP, our results being comparable to those previously reported using EMP alone.

Failure of chemotherapy to prolong life in patients with metastatic prostate cancer who have failed androgen deprivation

Using historical controls, we analyzed the effect of chemotherapy on survival of patients with metastatic prostate cancer who failed androgen deprivation. The survival of 178 patients with metastatic prostate cancer who receive palliation only after failing hormonal therapy was compared with the survival of 27 patients who received chemotherapy on NPCP Protocol 1500. Survival was measured from the initiation of androgen deprivation. The mean survival after chemotherapy was 8.4 months which was slightly less than the group receiving palliation only. At this time, chemotherapy has not been able to improve survival in patients with metastatic prostate cancer.

Androgen priming and response to chemotherapy in advanced prostatic cancer

A total of 67 patients with progressive stage D2 prostatic cancer refractory to orchiectomy was entered in a controlled clinical trial to test whether androgen priming enhances the efficacy of cytotoxic drugs. All patients were treated continuously with aminoglutethimide and hydrocortisone to lower adrenal androgen secretion and were given cyclic intravenous chemotherapy. In addition, the 34 patients randomized to the stimulation arm received fluoxymesterone for 3 days before and on the day of chemotherapy. There was 33 controls. The median duration of followup was 24 months. A modestly higher response rate (objective remission plus disease stabilization) was observed in the stimulation arm (85 versus 72 per cent, p less than 0.05) when the analysis was restricted to the evaluable patients. However, a larger fraction of unevaluable patients was present in the stimulation group (41 versus 16 per cent), mostly as a result of toxicity from fluoxymesterone, which prompted early discontinuation of treatment. Thus, when data analysis included all patients the response rate actually was slightly higher in the control than in the stimulation arm (60 versus 50 per cent, p not significant). No difference was observed in median duration of response (9 months in both groups) or over-all survival. Our data suggest that at least in those patients with advanced disease androgen priming does not seem to enhance significantly the antitumor effect of the combination of amino-glutethimide and chemotherapy, and is associated with significant toxicity. These largely negative results may be explained by the large number of hormone-resistant cells present in tumors that have become refractory to orchiectomy.

Preventing diversification of malignant tumor cells during therapy

One of the most difficult problems arising during cancer therapy is the emergence of unique tumor cell subpopulations that express altered malignant and therapeutic properties. In model tumor systems cytotoxic therapies that eliminate all but a few tumor cells seem to stimulate cell diversification to yield heterogeneous cell populations with divergent properties. Clinically the use of repeated cycles of cytotoxic therapies followed by intervening recovery periods often results in the eventual emergence of highly resistant and increasingly malignant tumor cells. To circumvent tumor cell diversification during therapeutic recovery periods we suggest that cytostatic agents might be employed. Although cytostatic agents such as differentiation modulators, hormones, growth factors, vitamins, and other natural products are unlikely to be effective therapeutic agents if used alone, they could be used during therapeutic recovery periods to modulate the diversification of surviving tumor cells so that these periods might be characterized by less extensive tumor cell diversification. At the end of recovery periods after the removal of cytostatic agents, surviving tumor cells could be briefly restimulated by growth factors, hormones or mitogens, just prior to initiation of new cycles of therapy using cytotoxic agents.

Do cells of continually renewing populations and those stimulated from quiescence respond similarly to HU and Ara-C?

The cytocidal and cytostatic effects of a single dose of either hydroxyurea (HU) or cytosine arabinoside (Ara-C) have been studied in a variety of tissues, from both rats and mice, by autoradiographic and histological methods. The necrogenic effects of the drugs on DNA-synthesising cells from continually renewing populations have been compared with the effects on cells induced to enter DNA synthesis from a formerly quiescent state. Four models of stimulated proliferation have been studied; the isoprenaline-stimulated salivary glands, the testosterone-stimulated seminal vesicle in the castrate, the regenerating liver after two-thirds partial hepatectomy, and a model of diethylnitrosamine (DEN)-induced hepatocyte proliferation. The time course of the cytokinetic response in each tissue was followed for a period of 24 h after drug treatment. The cytostatic effects of the drugs were similar in all tissues investigated, DNA synthesis being inhibited for a 4- to 5-h period. However, the sensitivity to the lethal effects showed both tissue and species variability.

Rationale for chemotherapeutic approaches to prostate cancer

The problems of curing tumors with a low growth fraction with cytotoxic chemotherapeutic agents include 1) the fact that such agents attack biochemical pathways common and vital to all cells, 2) the existence in a tumor of cells with both temporary and permanent resistance to specific agents, and 3) the exponential nature of cell kill, which necessitates a much more intensive treatment to effect cure than to effect remission. The possible bases for selectivity of anticancer drugs include those factors that affect the concentration of the active form of the drug at the active site, those that affect the drug receptor interaction and those that determine whether this interaction will lead to cell death. Possible ways of overcoming resistance include combination chemotherapy, more intensive chemotherapy, adjuvant chemotherapy, and maneuvers leading to recruitment of cells into cycle. Because of tumor cell heterogeneity a different approach may be needed to effect cure from that required to induce complete remission.

Chemotherapy principles in the treatment of prostatic cancer

The efficacy of chemotherapy for prostatic cancer is difficult to evaluate owing to the low incidence of measurable indicator lesions and the resulting need for indirect response criteria. Although complete regressions remain exceptional, a number of agents, eg, doxorubicin and cisplatin have been shown to be effective in the treatment of this disease. So far, combinations of effective agents with or without concomitant hormone therapy have not proven to be more effective than single agents. Androgen priming has considerable theoretical appeal and deserves further consideration. A higher effectiveness of chemotherapeutic agents might be obtained by linkage to various carriers. Estramustine phosphate is an example of such a complex that has a cytotoxic effect in test systems in which estrogen has no effect and in patients with hormone-refractory prostatic cancer. The use of hormonal and other carriers that could increase the specificity of chemotherapeutic agents deserves extensive exploration.

Drug resistance and cancer chemotherapy strategy in breast cancer

Evidence exists that spontaneous mutations to drug resistance are a major obstacle to chemotherapeutic cure in both experimental and clinical neoplasms. Analysis of this phenomenon indicates that certain treatment strategies may be more effective in circumventing resistance. Initiation of adjuvant chemotherapy at the earliest time feasible is such an approach. The use of combinations of non-cross resistant agents is another. These methods should reduce the probability that drug-resistant cells will be present at the time treatment is started, and in theory should produce improved therapeutic results.