Phase I trial of weekly paclitaxel plus oral estramustine phosphate in patients with hormone-refractory prostate cancer

Crosstalk between Microtubule Stabilizing Agents and Prostate Cancer

A variety of microtubule-stabilizing cytotoxic agents (MSA) with diverse chemical scaffolds have been discovered from marine sponges, microorganisms, and plants. Two MSAs, docetaxel and cabazitaxel, are the exclusive chemotherapeutics that convey a survival benefit in patients with castration-resistant prostate cancer (CRPC). Additional MSAs have been investigated for their potential in treating prostate cancer in both clinical and preclinical settings. Independent of promoting mitotic arrest, MSAs can suppress the nuclear accumulation of androgen receptor (AR), which is the driving force for prostate cancer cell growth and progression. The alternative mechanism not only helps to better understand the clinical efficacy of docetaxel and cabazitaxel for AR-driven CRPC but also provides an avenue to seek better treatments for various forms of prostate cancer. The dual mechanisms of action enable MSAs to suppress AR-null prostate cancer cell proliferation by cell mitosis pathway and to interfere with the AR signaling pathway in AR positive cells. MSA chemotherapeutics, being administered alone or in combination with other therapeutics, may serve as the optimal therapeutic option for patients with either castration-sensitive or castration-resistant prostate cancer. This review provides an overview of the anti-prostate cancer profiles (including preclinical and clinical studies, and clinical use) of diverse MSAs, as well as the mechanism of action.

Hormone Refractory Prostate Cancer (Hrpc): Present and Future Approaches of Therapy

The mainstay of therapy for patients with advanced prostate cancer still remains androgen deprivation, although response to this is invariably temporary. Most of the patients develop hormone-refractory disease resulting in progressive clinical deterioration and, ultimately, death. Until recently there has been no standard chemotherapeutic approach for hormone refractory prostate cancer (HRPC), the major benefits of chemotherapy being only palliative. The studies combining mitoxantrone plus a corticosteroid demonstrated that chemotherapy could be given to men with symptomatic HRPC with minimal toxicity and a significant palliation could be provided. Recently, results from 2 phase III randomized clinical trials demonstrating that a combination of docetaxel plus prednisone can improve survival in men with HRPC have propelled docetaxel-based therapy into the forefront of treatment options for these patients as the new standard of care. There is a promising activity of new drug combinations such as taxanes plus vinca alkaloids; bisphosphonates are assuming a prominent role in prostate therapy through their ability to prevent skeletal morbidity. Combinations of classic chemotherapeutic agents and biological drugs began to be tested in phase II-III trials and the first results appear interesting. This article focuses on combinations recently evaluated or under clinical development for the treatment of HRPC.

Is There a Standard Chemotherapeutic Regimen for Hormone-Refractory Prostate Cancer? Present and Future Approaches in the Management of the Disease

Prostate cancer that no longer responds to hormonal manipulation can be defined as hormone-refractory prostate cancer. Until recently, there has been no standard chemotherapeutic approach for hormone-refractory prostate cancer. The major benefits of chemotherapy in the treatment of the disease are palliative in nature, in terms of reduction of pain and use of analgesics and improvement of performance status, as followed in the most recent trials. Phase III studies are necessary to better evaluate the efficacy of the different regimens, because several old studies suffer for methodological deficits. There is a promising activity of new drug combinations, such as vinca alkaloids and taxanes. Phase I and II trial are testing combinations of classic chemotherapeutic agents and biologic drugs, and the first results appear interesting. In this article, recent advances in the treatment of hormone-refractory prostate cancer using chemotherapeutic regimens are critically reviewed.

Phase 2 Study of Weekly Paclitaxel Plus Estramustine in Metastatic Hormone-Refractory Prostate Carcinoma: ECOG-ACRIN Cancer Research Group (E1898) Trial

INTRODUCTION

This multicenter phase 2 study assessed the combination of estramustine and weekly paclitaxel with metastatic castration-resistant prostate cancer (CRPC).

PATIENTS AND METHODS

We enrolled 77 patients who had received no prior chemotherapy for CRPC between 1998 and 2000; a total of 74 subjects were eligible for the study. Each 8-week cycle included paclitaxel 90 mg/m² provided intravenously weekly for 6 weeks, followed by 2 weeks off therapy and oral estramustine 280 mg twice daily for 3 days beginning 24 hours before the first dose of paclitaxel. The primary end point was rate of objective or prostate-specific antigen (PSA) response at 16 weeks. A 50% response rate was considered of further interest.

RESULTS

Eligible patients received a median of 3 cycles (range, 1-10 cycles). The response rate among patients with measurable disease was 34% (95% confidence interval [CI], 19-52). The PSA response rate was 58% (95% CI, 47-70). Clinical benefit rate was 45% (95% CI, 33-57). The median progression-free survival was 5.9 months (95% CI, 4.4-6.7). The median overall survival was 17.6 months (95% CI, 14.6-20.8). The most common clinical grade 3/4 toxicities were fatigue (14%) and sensory neuropathy (7%). Grade 3/4 hematologic toxicities included lymphopenia (21%) and anemia (9%). There was one toxicity-related death. Quality-of-life scores improved by week 8, but the change was not statistically significant.

CONCLUSION

The combination has activity defined by PSA declines in CRPC but did not meet the protocol-specified end point for efficacy as defined by objective response rate. Since this study was conducted, more effective, better-tolerated regimens have been developed.

Number of Patients per Cohort and Sample Size Considerations Using Dose Escalation with Overdose Control

The main objective of cancer phase I clinical trials is to determine a maximum tolerated dose (MTD) of a new experimental treatment. In practice, most of these trials are designed so that three patients per cohort are treated at the same dose level. In this paper, we compare the safety and efficiency of trials using the escalation with overdose control (EWOC) scheme designed with three or only one patient per cohort. We show through simulations that the number of patients per cohort does not impact the proportion of patients given therapeutic doses, safety of the trial, and efficiency of the estimate of the MTD. Additionally, we present guidelines and tabulated values on the number of patients needed to design a phase I cancer clinical trial using EWOC to achieve a given accuracy of the estimate of the MTD.

Bone-targeted therapy: phase II study of strontium-89 in combination with alternating weekly chemohormonal therapies for patients with advanced androgen-independent prostate cancer

OBJECTIVES

Bone-targeted therapy that combines strontium-89 (Sr-89) with alternating weekly chemohormonal therapy may improve clinical outcomes in patients with metastatic hormone-refractory prostate cancer. This phase II study investigated the addition of Sr-89 to an alternating weekly regimen of doxorubicin and ketoconazole with paclitaxel and estramustine in patients with progressive prostate cancer and bone involvement.

METHODS

Twenty-nine patients with progressive adenocarcinoma of the prostate and osteoblastic bone metastases who failed conventional hormonal therapy were registered for the study. Of those, 27 were treated with Sr-89 on day 1 of week 1. On weeks 1, 3, and 5, patients received doxorubicin (20 mg/m on day 1) and oral ketoconazole (400 mg 3 times a day for 7 days). On weeks 2, 4, and 6, patients received paclitaxel (100 mg/m(2)) and oral estramustine (280 mg 3 times a day for 7 days). No treatment was given during weeks 7 and 8. Cycles were repeated every 8 weeks.

RESULTS

A > or =50% reduction in prostate-specific antigen level was maintained for at least 8 weeks in 77.7% of the patients (21 patients) at 16 weeks and in 66.6% (18 patients) at 32 weeks. The median progression-free survival was 11.27 months (range, 1.83-29.53), and the median overall survival was 22.67 months (1.83-57.73+). Two patients died during study because of disease progression. Overall, the chemotherapy combined with Sr-89 was well tolerated.

CONCLUSIONS

Our results demonstrate that the combination of alternating weekly chemohormonal therapies with Sr-89 demonstrates a prolonged progression-free and overall survival with acceptable toxicity. Further investigation of combination therapies with Sr-89 is warranted.

New standards in the chemotherapy of metastatic hormone-refractory prostate cancer

Hormone-refractory prostate cancer (HRPC) is a major issue in Western countries and the second leading cause of cancer death in North American men. In the prostate-specific antigen era, most HRPCs are currently diagnosed in asymptomatic patients based on biochemical failure, with increasing demand for active treatment. Until recently, chemotherapy for HRPC patients was not considered a standard of care due to the absence of clear data evidencing an overall survival benefit. In fact, few Phase III studies conducted in the 1980s and early 1990s had documented a superiority over corticosteroids alone in terms of biochemical response (declines in serum prostate-specific antigen levels) and quality of life, but not survival. Due to their impact on pain control, mitoxantrone and prednisone were long considered the best regimen for symptomatic HRPC patients. In recent years, more chemotherapeutic agents have been tested, among which the microtubule inhibitors (vinca alkaloids and taxanes) have obtained the most promising results in Phase II trials and have entered Phase III testing. Two well-designed randomized trials have changed this scenario. Both compared docetaxel (with or without estramustine) against mitoxantrone and prednisone, and demonstrated a significant advantage not only in terms of response, pain control and quality of life, but also in terms of overall survival. Which patients need to be treated, the regimen of choice and duration of chemotherapy will be the next questions to be answered in the coming years in the field of HRPC, along with the role of new signal transduction inhibitors and other targeted therapies.

Cancer of the prostate

Prostate carcinoma, with about 190,000 new cases occurring each year (15% of all cancers in men), is the most frequent cancer among men in northern and western Europe. Causes of the disease are essentially unknown, although hormonal factors are involved, and diet may exert an indirect influence; some genes, potentially involved in hereditary prostate cancer (HPC) have been identified. A suspect of prostate cancer may derive from elevated serum prostate-specific antigen (PSA) values and/or a suspicious digital rectal examination (DRE) finding. For a definitive diagnosis, however, a positive prostate biopsy is requested. Treatment strategy is defined according to initial PSA stage, and grade of the disease and age and general conditions of the patient. In localized disease, watchful waiting is indicated as primary option in patients with well or moderately differentiated tumours and a life expectancy <10 years, while radical prostatectomy and radiotherapy (with or without hormone-therapy) could be appropriate choices in the remaining cases. Hormone-therapy is the treatment of choice, combined with radiotherapy, for locally advanced or bulky disease and is effective, but not curative, in 80-85% of the cases of advanced disease. Patients who develop a hormone-refractory prostate cancer disease (HRPC) have to be evaluated for chemotherapy because of the recent demonstration of improved overall survival (2-2.5 months) and quality of life with docetaxel in more than 1,600 cases.

Synergistic inhibitory effect of high-intensity focused ultrasound combined with chemotherapy on Dunning adenocarcinoma

OBJECTIVE

To evaluate the therapeutic effect of high-intensity focused ultrasound (HIFU) combined with chemotherapy (paclitaxel + estramustine) on AT2 Dunning adenocarcinoma, as no satisfactory treatment for localized prostate cancer is available for patients with a poor prognosis, e.g. stage T3, a high Gleason score, or a prostate-specific antigen level of >15 ng/mL.

MATERIALS AND METHODS

Forty-one Dunning AT2 tumour-bearing Copenhagen rats were divided into four groups, i.e. control, chemotherapy, HIFU, and chemotherapy + HIFU (the last three treated for 1 week). The growth in tumour volume was recorded for 3 weeks, the point at which tumour volume was considered to have doubled (doubling time). The growth curves of each group were plotted and evaluated statistically.

RESULTS

At 30 days of follow-up the distributions of tumour volume with treatment group were significantly different (P < 0.001); volumes were significantly greater in the control than in the chemotherapy-only or in the HIFU-only group (both P = 0.006). The greatest difference was between the chemotherapy + HIFU and the control group. The tumour doubling times were 13.2 days for HIFU-only, 31.2 days for chemotherapy + HIFU and 7.7 days for the controls.

CONCLUSION

These results suggest that this combined therapy could be useful for treating patients with high-risk prostate cancer.

Phase II Randomized Trial of Weekly Paclitaxel with or Without Estramustine Phosphate in Progressive, Metastatic, Hormone-Refractory Prostate Cancer

This study was conducted to determine the similarity of response rates and safety produced by weekly paclitaxel with or without oral estramustine in patients with metastatic hormone-refractory prostate cancer. Between December 1998 and December 1999, 163 patients were randomized to receive 28-day cycles of paclitaxel 100 mg/m2 on days 2, 9, and 16 plus estramustine 280 mg orally 3 times a day on days 1-3, 8-10, and 15-17, or to receive paclitaxel 100 mg/m2 alone on days 1, 8, and 15. Objective response was defined as a > oe = 50% decrease in prostate-specific antigen (PSA) maintained for 4 weeks with stable or improved performance status. Response rates included 37 partial responses for paclitaxel/estramustine (47%) and 22 partial responses for paclitaxel (27%; P < 0.01). Median duration of response was 15.1 months for paclitaxel/estramustine and 15.5 months for paclitaxel; median survival was 16.1 months and 13.1 months, respectively (P = 0.049). Common toxicities for both treatments included neutropenia, gastrointestinal events, neuropathy, and asthenia. Thromboembolic events were more frequent in the paclitaxel/estramustine arm (no prophylactic anticoagulants). The rate of PSA decline for paclitaxel/estramustine was almost 2 times that of paclitaxel (47% vs. 27%), with acceptable toxicity. Multivariate analysis of prognostic factors affecting survival was not significant for treatment arm (P = 0.08). Although the incidence of thromboembolic events appeared to be increased in the paclitaxel/ estramustine arm, the addition of estramustine was responsible for a 20% increase in the rate of PSA decline. Neither treatment arm had significant impact on quality of life as measured by the Functional Assessment of Cancer Therapy-Prostate quality of life questionnaire. This study produced encouraging data; further studies of paclitaxel/ estramustine are recommended.

[Taxanes in the chemotherapy of hormone-refractory prostate carcinoma]

Prostate cancer represents one of the most prevalent malignancies in men. Standard therapy of metastatic prostate cancer consists of androgen deprivation, which is a palliative therapy yielding a clinical response of limited duration. In hormone-refractory prostate cancer (HRPC), response to chemotherapy with regimens available until about ten years ago has been disappointing. Nowadays, due to increasing life expectancy and earlier diagnosis and therapy of prostate cancer, more patients with hormone-refractory disease are still in relatively good overall condition. With the taxanes, much more effective cytostatic substances for chemotherapy of HRPC are available today. Using modern taxane-based chemotherapy, effective palliation of pain can be achieved in 50-70% of patients with HRPC, while retaining an acceptable quality of life. There is also evidence for improved overall survival after taxane-based chemotherapy, although this remains to be proven by ongoing studies. This article presents an overview of current studies investigating the outcome after taxane-based chemotherapy, as well as new therapeutic approaches in combination with docetaxel.

[Chemotherapy of hormone refractory prostate carcinoma]

An increasing life expectancy and the growing number of largely healthy older men have lead to more patients with hormone insensitive relapses after palliative hormone or curative therapy for prostate cancer. After 10 years without therapeutic improvement for hormone refractory prostate cancer, the introduction of new substances has led to a revival of chemotherapy. Although a definitive cure is still not possible, such chemotherapy fulfils important palliative criteria-good toleration and an improvement in quality of life-in addition to distinct long-term remission. For example, taxane as a monotherapy or in combination with estramustine is effective and well tolerated while mitoxantrone in combination with prednisolone, although of limited effectiveness, leads to a substantial reduction in symptoms. Although evidence for increased longevity through modern chemotherapy is available, this has still not been definitively demonstrated. The substantial reduction in pain and therapy related morbidity frequently makes chemotherapy for hormone refractive prostate cancer a superior alternative to simple pain and complication management.

Vinblastine and estramustine phosphate in metastatic renal cell carcinoma

BACKGROUND

It is well known that metastatic renal cell carcinoma (RCC) exhibits constitutive resistance to chemotherapeutic agents. Antimicrotubule agents such as vinblastine are associated with low but reproducible response rates (approximately 12%) in patients with RCC. Estramustine has been shown to potentiate the antimicrotubule effects of vinblastine. The authors sought to increase the activity of vinblastine in RCC through the addition of estramustine.

METHODS

Twenty-one patients with metastatic RCC not previously treated with chemotherapy received oral estramustine phosphate, 600 mg/m(2), on Days 1, 2, and 3 weekly for 6 weeks, and intravenous vinblastine, 4 mg/m(2) on Day 2 weekly for 6 weeks, repeated every 8 weeks. Twenty-one patients received 31 cycles of therapy.

RESULTS

Two patients experienced Grade 3 and 4 hematologic toxicity, and three patients had Grade 3 nonhematologic toxicity consisting of neurologic toxicity, hepatic toxicity, or angioneurotic edema. One patient had a partial response with decreased liver metastases for 48 weeks; 9 patients had stable disease, for a median duration of 14 weeks (range, 11-31 weeks); and 11 patients demonstrated disease progression. The median overall time to progression was 8 weeks and the median overall survival period was 24 weeks.

CONCLUSIONS

Although well tolerated, the combination of oral estramustine phosphate with vinblastine administered on this schedule had minimal activity in patients with metastatic RCC.

Weekly paclitaxel plus estramustine combination therapy in hormone-refractory prostate cancer: a pilot study

BACKGROUND

Paclitaxel used in combination with estramustine has been shown to exert synergistic cytotoxicity in patients with hormone-refractory prostate cancer (HRPC). There have been few reports of this therapy in an Asian male population.

METHODS

Nine patients with progressive metastatic HRPC completed at least one cycle of combination therapy employing weekly paclitaxel plus estramustine. Paclitaxel was given weekly for 3 weeks as a 2-h intravenous infusion at a dose of 100 mg/infusion. The cycle was repeated every 4 weeks. A dose of 280 mg of oral estramustine was administrated twice daily for 21 days from the first day of each cycle. Both efficacy and toxicity were recorded.

RESULTS

Grade 1 sensory neuropathy was seen in three patients (33%) and grade 4 thrombopenia/anemia was seen in one patient (11%). Performance status improved in three of seven patients (43%), while six patients (67%) showed a 50% or greater decline in prostate-specific antigen levels. Two of these patients experienced significant improvement in bone pain. One patient died of cardiac infarction during this trial and another died of disseminated intravascular coagulopathy subsequent to gastrointestinal bleeding. An additional patient suffered non-fatal pulmonary infarction. The one-year median survival rate was 22.2% and the overall survival period was 36 weeks.

CONCLUSION

Although weekly paclitaxel plus estramustine may pose a significant risk, this combination may have a beneficial effect on the quality of life HRPC patients. A well-designed phase I-II trial in an Asian male population is highly recommended.

Current strategies in the management of hormone refractory prostate cancer

Prostate cancer is the most common cancer diagnosed in American males, and is the second leading cause of cancer-related deaths. Most patients who develop metastatic disease will initially respond to androgen deprivation, but response is invariably temporary. Most patients will develop androgen-independent ("hormone-refractory") disease that results in progressive clinical deterioration and ultimately death. This progression to androgen independence is accompanied by increasingly evident DNA instability and alterations in genes and gene expression, including mutations in p53, over-expression of Bcl2, and mutations in the androgen receptor gene, among others. Treatment options for hormone refractory disease include intensive supportive care, radiotherapy, bisphosphonates, second-line hormonal manipulations, cytotoxic chemotherapy and investigational agents. A post-treatment reduction in the level of prostate specific antigen (PSA) by 50% has been shown to correlate with survival and has been accepted by consensus as a valid endpoint in clinical trials. Chemotherapeutic agents such as mitoxantrone, estramustine, and the taxanes have yielded improved response rates and palliative benefit, but not improved survival. Therefore, current efforts must be focused on enrolling patients onto clinical trials of investigational agents with novel mechanisms of action, and on using survival, time to progression, and quality of life as end points in routine clinical practice.

Management of high-risk localized prostate cancer: the integration of local and systemic therapy approaches

Using a combination of PSA, Gleason score, and clinical stage, it is possible to identify a group of patients with prostate cancer who have a high risk of relapse following initial treatment (e.g., radiotherapy or radical prostatectomy). For these patients, multi-modal therapy may result in improved outcomes. We reviewed published literature to identify methods to identify high-risk patients as well as options for adjuvant or neoadjuvant therapy to reduce risk of disease recurrence. At the present time, the most promising adjuvant therapy is hormonal therapy following radiotherapy for locally advanced disease (T3-T4, or N1). In phase III trials in these patients, survival is improved. For all other applications, including adjuvant and neoadjuvant hormonal therapy, chemotherapy, or radiotherapy, the benefits are unclear. Perhaps most promising at this time, and the subject of a current phase III trial, is the utility of adjuvant chemotherapy in high-risk patients. It will be through the conduct of phase III trials that the benefits of multi-modal therapy will be evaluated. Patients with high-risk prostate cancer undergoing radiotherapy or surgery should be offered participation in these clinical trials.

Activity of docetaxel with or without estramustine phosphate versus mitoxantrone in androgen dependent and independent human prostate cancer xenografts

PURPOSE

Prostate cancer is known to be refractory to chemotherapy with only mitoxantrone showing some benefit. Recent clinical data indicate the antitumoral efficacy of taxanes alone or combined with estramustine phosphate. We compared the response to these treatments of hormone dependent and independent human prostate cancer xenografts.

MATERIALS AND METHODS

PAC120, an androgen dependent human prostate cancer xenograft, and several HIDs, which are androgen independent variants, were established in nude mice. Human gene expression was determined by semiquantitative reverse transcriptase-polymerase chain reaction. Androgen deprivation was achieved by surgical castration. Tumor bearing mice received 20 mg./kg. docetaxel on day 2, 1 mg./kg. hydrocortisone on days 1 to 3, 4 mg./kg. estramustine phosphate on days 1 to 4 or 1 mg./kg. mitoxantrone on day 1 by the intraperitoneal route for 3-week cycles. Relative tumor volume and growth delay were evaluated. Histological examination of tumors was done before and after treatment.

RESULTS

Mitoxantrone transiently decreased the growth rate of HID xenografts but did not affect that of PAC120. Estramustine phosphate alone inhibited the growth of PAC120 but not that of HID variants. Docetaxel inhibited the growth of all prostate cancer xenografts (PAC120 and HIDs) and increased survival. PAC120 showed distinct response patterns during prolonged treatment. Efficacy was significantly decreased by splitting docetaxel into 2 doses given at a 7-day interval (p = 0.01). The docetaxel effect was potentiated by estramustine phosphate in 1 of the 2 HID variants tested. In castrated mice docetaxel induced a greater growth delay than in intact male mice (p = 0.01). High Her2/neu and beta2-tubulin transcripts were detected in all samples. Prostate specific antigen, androgen receptor and multidrug related protein-1 mRNA did not correlate with the drug response, while CYP3A4 mRNA inversely correlated with the response. Docetaxel treated tumors had an increased number of mitotic cells with centrosome alterations and multinuclei, an increased number of Ki67 labeled cells and a strong decrease in beta-tubulin without evidence of apoptosis.

CONCLUSIONS

Docetaxel showed a significant antitumoral effect on hormone dependent and tumors, which was largely superior to that of mitoxantrone. Estramustine phosphate alone had a modest effect. The drug response was associated with high Her2/neu expression, low CYP3A4 expression and the induction of numerous mitotic abnormalities.

Combination chemotherapy with paclitaxel, estramustine and carboplatin for hormone refractory prostate cancer

PURPOSE

The activity of estramustine phosphate is synergistic with paclitaxel against hormone refractory prostate cancer. Moreover, the single agent activity of carboplatin has demonstrated a 17% response rate in measurable disease. Therefore, we conducted a prospective trial to establish more effective chemotherapy consisting of paclitaxel, estramustine phosphate and carboplatin for hormone refractory prostate cancer.

MATERIALS AND METHODS

The study included 32 patients with hormone refractory prostate cancer. Prior chemotherapy was accepted. Patients were treated with 100 mg./m.2 paclitaxel intravenously weekly, 10 mg./kg. estramustine phosphate orally daily and carboplatin intravenously to an area under the curve of 6 on day 1 of every 4-week cycle. Treatment was continued until disease progression or excessive toxicity.

RESULTS

Of the 32 patients 30 were assessable for response. A median of 7 consecutive cycles was administered per patient. Ten patients had received prior cytotoxic chemotherapy. Levels of prostate specific antigen decreased by greater than 50% in 100% of patients and by greater than 90% in 56.7%. Partial response was obtained in 61.1% of measurable lesions. Consumption of medication for cancer induced pain was reduced in 89.5% of patients. Tumor volume reduction and/or antitumor therapeutic effects were exhibited in 81.0% of patients with positive biopsy. At a median followup of 48 weeks median time to progression was 48 weeks and median overall survival was 95 weeks. Two patients suffered myocardial infarction and hepatic insufficiency, respectively, and discontinued treatment during the first cycle. Major toxicities were grade 3 or 4 anemia in 59.4% of patients, leukopenia in 37.5%, thrombocytopenia in 28.1% and neuropathy in 12.5%. However, all toxicity was temporary and reversible with dose reduction or temporary cessation of chemotherapeutic agents.

CONCLUSIONS

Paclitaxel, estramustine phosphate and carboplatin chemotherapy was extremely effective for hormone refractory prostate cancer. Although hematological and neurotoxicity were modest, this therapy may be more manageable with lower doses.

Reversal of P-glycoprotein-mediated paclitaxel resistance by new synthetic isoprenoids in human bladder cancer cell line

We isolated a paclitaxel-resistant cell line (KK47/TX30) from a human bladder cancer cell line (KK47/WT) in order to investigate the mechanism of and reversal agents for paclitaxel resistance. KK47/TX30 cells exhibited 700-fold resistance to paclitaxel and cross-resistance to vinca alkaloids and topoisomerase II inhibitors. Tubulin polymerization assay showed no significant difference in the ratio of polymerized alpha- and beta-tubulin between KK47/WT and KK47/TX30 cells. Western blot analysis demonstrated overexpression of P-glycoprotein (P-gp) and lung resistance-related protein (LRP) in KK47/TX30 cells. Drug accumulation and efflux studies showed that the decreased paclitaxel accumulation in KK47/TX30 cells was due to enhanced paclitaxel efflux. Cell survival assay revealed that verapamil and cepharanthine, conventional P-gp modulators, could completely overcome paclitaxel resistance. To investigate whether new synthetic isoprenoids could overcome paclitaxel resistance, we synthesized 31 isoprenoids based on the structure of N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine (SDB), which could reverse multidrug resistance (MDR), as shown previously. Among those examined, trans-N,N'-bis(3,4-dimethoxybenzyl)-N-solanesyl-1,2-diaminocyclohexane (N-5228) could completely reverse paclitaxel resistance in KK47/TX30 cells. N-5228 inhibited photoaffinity labeling of P-gp by [(3)H]azidopine, suggesting that N-5228 could bind to P-gp directly and could be a substrate of P-gp. Next, we investigated structural features of these 31 isoprenoids in order to determine the structural requirements for the reversal of P-gp-mediated paclitaxel resistance, suggesting that the following structural features are important for overcoming paclitaxel resistance: (1) a basic structure of 8 to 10 isoprene units, (2) a cyclohexane ring or benzene ring within the framework, (3) two cationic sites in close proximity to each other, and (4) a benzyl group with 3,4-dimethoxy functionalities, which have moderate electron-donating ability. These findings may provide valuable information for the development of P-gp-mediated MDR-reversing agents.

EMP combination chemotherapy and low-dose monotherapy in advanced prostate cancer

Many chemotherapeutic regimens combined with estramustine phosphate (EMP) have been elaborated for the treatment of hormone-refractory prostate cancer over 30 years. However, older EMP-based combination chemotherapies with vinblastine, vinorelbine, doxorubicin or cyclophosphamide showed relatively low PSA response rate (25-58%) accompanied with high toxicities. On the other hand, newly developed EMP-based combination regimens with etoposide, pacitaxel, carboplatin or docetaxel demonstrated promising PSA response rate (43-77%) with moderate to severe toxicity in the rate of thromboembolic event (5-18%) and of neutropenia (9-41%). Treatment-related death was less in the latter combination group (5/615, 0.8%) than that in the former group (3/234, 1.3%). Of note, in the docetaxel combination with EMP, PSA response rate is as high as 77% with high rate (41%) of neutropenia but no treatment-related death was observed. Docetaxel combination with EMP seems to be the best regimen, though not completely justified by randomized trials, to be selected in the modern era, which will be followed by paclitaxel, carboplatin and EMP combination with PSA response rate of 71%. In addition, an interim report in 83 patients was presented. They were not consecutively enrolled but were treated on low-dose EMP monotherapy for previously untreated advanced prostate cancer in Department of Urology of Tokyo University and our 21 affiliated hospitals. Overall PSA response rate was as high as 93.4% out of 76 assessable patients. However, overall toxicity rate was abnormally high (39.5%) with drug discontinuation rate of 32.1%. The reason of low drug compliance may be attributed to gastrointestinal symptoms. To overcome the low drug compliance, appropriate patients for EMP administration should be selected by using gene analysis on the basis of sophisticated tailor-made medicine.

Paclitaxel and docetaxel in prostate cancer

Although their ultimate value in prostate cancer therapy remains to be defined in randomized trials, docetaxel and paclitaxel are active agents in HRPC. Combination therapies using either of these taxanes plus oral EMP show reproducible antitumor activity that appears to be greater and more durable than that of single-agent treatment. Although the optimal combination and schedule have not been determined, weekly paclitaxel and EMP and docetaxel given every 3 weeks or by weekly infusion with EMP are useful treatment options for patients with progressive HRPC. The gastrointestinal toxicity of EMP has been reduced by intermittent rather than continuous administration, and other toxicities may be reduced further by use of intravenous EMP. Although there has been progress, the median time to progression of 5 to 6 months for current taxane-based therapies suggests that they will not have major impact on overall survival for patients with HRPC. Greater benefit may be possible earlier in the course of prostate cancer, and the activity of the taxane-EMP combinations is sufficient to justify clinical trials of adjuvant or neoadjuvant chemotherapy for selected groups of patients with locally advanced and poor-prognosis tumors. Armed with many new molecularly targeted agents that may interact favorably with taxanes, it should be possible to build on current antimicrotubule regimens to improve activity in HRPC. Taxane-EMP combinations provide a platform on which to test additional agents that may enhance the apoptotic response or circumvent cellular stress adaptations that confer drug resistance. Further elucidation of signaling pathways that regulate microtubule dynamics and programmed cell death after exposure to microtubule inhibitors would provide a more rational guide for integrating specific inhibitors of signal transduction with current taxane-based therapies. Pharmacokinetic and pharmacodynamic studies will play a key role in the development of future taxane-based therapies for prostate cancer.