Pathways of chemotherapy resistance in castration-resistant prostate cancer

Chemotherapy remains the major treatment option for castration-resistant prostate cancer (CRPC) and limited cytotoxic options are available. Inherent chemotherapy resistance occurs in half of all patients and inevitably develops even in those who initially respond. Docetaxel has been the mainstay of therapy for 6 years, providing a small survival benefit at the cost of significant toxicity. Cabazitaxel is a promising second-line agent; however, it is no less toxic, whereas mitoxantrone provides only symptomatic benefit. Multiple cellular pathways involving apoptosis, inflammation, angiogenesis, signalling intermediaries, drug efflux pumps and tubulin are implicated in the development of chemoresistance. A thorough understanding of these pathways is needed to identify biomarkers that predict chemotherapy resistance with the aim to avoid unwarranted toxicities in patients who will not benefit from treatment. Until recently, the search for predictive biomarkers has been disappointing; however, the recent discovery of macrophage inhibitory cytokine 1 as a marker of chemoresistance may herald a new era of biomarker discovery in CRPC. Understanding the interface between this complex array of chemoresistance pathways rather than their study in isolation will be required to effectively predict response and target the late stages of advanced disease. The pre-clinical evidence for these resistance pathways and their progress through clinical trials as therapeutic targets is reviewed in this study.

Angiogenesis as a strategic target for prostate cancer therapy

It is becoming increasingly clear that angiogenesis plays a crucial role in prostate cancer (CaP) survival, progression, and metastasis. Tumor angiogenesis is a hallmark of advanced cancers and an attractive treatment target in multiple solid tumors. By understanding the molecular basis of resistance to androgen withdrawal and chemotherapy in CaP, the rational design of targeted therapeutics is possible. This review summarizes the recent advancements that have improved our understanding of the role of angiogenesis in CaP metastasis and the potential therapeutic efficacy of inhibiting angiogenesis in this disease. Current therapeutic options for patients with metastatic hormone-refractory CaP are very limited. Targeting vasculature is a developing area, which shows promise for the control of late stage and recurrent CaP disease and for overcoming drug resistance. We discuss angiogenesis and its postulated mechanisms and focus on the regulation of angiogenesis in CaP progression and the therapeutic beneficial effects associated with targeting of the CaP vasculature to overcome the resistance to current treatments and CaP recurrence.

A phase II clinical trial of sorafenib in androgen-independent prostate cancer

PURPOSE

To determine if sorafenib is associated with a 4-month probability of progression-free survival, which is consistent with 50%, as determined by clinical, radiographic, and prostate-specific antigen (PSA) criteria in patients with metastatic androgen-independent prostate cancer (AIPC).

EXPERIMENTAL DESIGN

Patients with progressive metastatic AIPC were enrolled in an open-label, single-arm phase II study. Sorafenib was given continuously at a dose of 400 mg orally twice daily in 28-day cycles. Clinical assessment and PSA measurement were done every cycle whereas radiographic measurements were carried out every two cycles.

RESULTS

Twenty-two patients were enrolled in the study to date, completing a planned first stage of the trial. Baseline patient characteristics included a median age of 63.9 years (range, 50-77 years), Gleason score of 9 (range, 4-9.5), and PSA concentration of 53.3 ng/mL (range, 2-1,905 ng/mL). Fifty-nine percent of patients had received one prior chemotherapy regimen. Of the 21 patients with progressive disease, 13 progressed only by PSA criteria in the absence of evidence of clinical and radiographic progression. Two patients were found to have dramatic reduction of bone metastatic lesions as shown by bone scan, although they met PSA progression criteria at the time when scans were obtained. Toxicities likely related to treatment included one grade 3 hypertension; one grade 3 hand-foot syndrome; and grade 1/2 toxicities: fatigue, anorexia, hypertension, skin rash, nausea, and diarrhea. Results from in vitro studies suggested that PSA is not a good marker of sorafenib activity. The geometric mean exposure (AUC(0-12)) and maximum concentration (C(max)) were 9.76 h mg/L and 1.28 mg/L, respectively. The time to maximum concentration (t(max)) and accumulation ratio (after second dose) ranged from 2 to 12 h and 0.68 to 6.43, respectively.

CONCLUSIONS

Sorafenib is relatively well tolerated in AIPC with two patients showing evidence of improved bony metastatic lesions. Interpretation of this study is complicated by discordant radiographic and PSA responses. PSA may not be an adequate biomarker for monitoring sorafenib activity. Based on these observations, further investigation using only clinical and radiographic end points as progression criteria is warranted. Accrual to the second stage of trial is ongoing.

[Promising new treatment options for metastatic androgen-independent prostate cancer]

OBJECTIVE

Review the recent advances in the treatment of androgen independent prostate cancer (AIPC).

METHODS

Review recent abstracts and literature utilizing Medline/PubMed using key words: androgen independent/hormone refractory prostate cancer, novel treatment options, Phase II, III trials and meeting abstracts/presentations.

CONCLUSION

Two pivotal trials SWOG (Southwest Oncology Group) study 9916 and Taxotere 327 have shown that survival can be improved in this population by administration of chemotherapy with docetaxel every three weeks intravenously. An overall survival of 19 months could be achieved with docetaxel/prednisone compared to 16 months with mitoxantrone/prednisone. Despite this, there is a need to improve on this survival benefit because the relapse free survival among responders is often short (6 months) and patients often would have progression of their cancer leading to death. Satraplatin, a novel platinum analogue had been found to provide an additional 1.5 week progression free survival benefit in this population in the second line setting. There is however, a need to develop less toxic drugs that would improve survival significantly.

A phase II study of sorafenib in patients with chemo-naive castration-resistant prostate cancer

BACKGROUND

The purpose of this trial was to evaluate the antitumor activity of sorafenib, a multikinase inhibitor of cell proliferation and angiogenesis, in patients with castration-resistant prostate cancer.

PATIENTS AND METHODS

This was a multicenter, two-stage, phase II study. Sorafenib 400 mg was administered orally twice daily continuously. Primary end point was prostate-specific antigen (PSA) 'response' defined as a > or =50% decrease for > or =4 weeks.

RESULTS

In all, 28 patients were enrolled. Eastern Cooperative Oncology Group performance status was zero or one in 19 and 9 patients. Two patients had no metastases, and 26 had bone and/or lymph node disease. A median of two cycles (range 1-8) was delivered. Adverse events were typical for sorafenib. The PSA response rate was 3.6% [95% confidence interval (CI) 0.1% to 18.3%] with response occurring in one patient (baseline = 10 000 and nadir = 1643 microg/l). No measurable disease responses occurred in eight patients. Time to PSA progression was 2.3 months (95% CI 1.8-6.4). Of 16 patients who discontinued sorafenib and then did not receive any immediate therapy, 10 had postdiscontinuation PSA declines of 7%-52%.

CONCLUSIONS

Sorafenib has limited activity using current PSA criteria. The declines in PSA observed on treatment discontinuation indicate an effect on PSA production/secretion. Further study may be warranted but needs to consider the limitations of PSA as an indicator of progression and response.

Phase I dose escalation and pharmacokinetic study of AZD2171, an inhibitor of the vascular endothelial growth factor receptor tyrosine kinase, in patients with hormone refractory prostate cancer (HRPC)

To explore the pharmacokinetics and tolerability of AZD2171, an inhibitor of vascular endothelial growth factor receptors 1 and 2, in patients with hormone refractory prostate cancer. Twenty-six patients received oral daily dosing of AZD2171 at 1, 2.5, 5, 10, 20, 30 mg. The maximum tolerated dose (MTD) was defined as the dose below that at which >or=33% of patients experienced a dose-limiting toxicity (DLT) within 21 days of initiating therapy. Pharmacokinetic analysis was performed. DLTs occurred at the 30 mg dose and included grade 3 events in three patients: fatigue (n = 3) and muscle weakness (n = 2). The pharmacokinetic profile revealed an effective half-life of approximately 27 h. At steady state, the unbound drug concentration was 4.4 times above the concentration required to inhibit endothelial cell proliferation in vitro. Four patients experienced PSA reductions within 30 days following drug discontinuation (one on 2.5 mg, two on 20 mg and 1 on 30 mg). In two patients treated with 20 mg, post therapy PSA declines persisted for >17 months, despite a PSA increase on therapy. Resolution of adenopathy occurred in one patient persisting for >17 months. Plasma concentrations were maximum 2-8 h post dosing with an overall median value of 2 h. The dose of 20 mg daily was declared as the MTD. One objective response and several PSA declines following the discontinuation of therapy for toxicity suggest that evidence of clinical efficacy may be delayed. While further study is indicated, careful attention must be paid to the novel toxicities of this agent with prolonged dosing.

Discovery and development of sorafenib: a multikinase inhibitor for treating cancer

Since the molecular revolution of the 1980s, knowledge of the aetiology of cancer has increased considerably, which has led to the discovery and development of targeted therapies tailored to inhibit cancer-specific pathways. The introduction and refinement of rapid, high-throughput screening technologies over the past decade has greatly facilitated this targeted discovery and development process. Here, we describe the discovery and continuing development of sorafenib (previously known as BAY 43-9006), the first oral multikinase inhibitor that targets Raf and affects tumour signalling and the tumour vasculature. The discovery cycle of sorafenib (Nexavar; Bayer Pharmaceuticals) - from initial screening for a lead compound to FDA approval for the treatment of advanced renal cell carcinoma in December 2005 - was completed in just 11 years, with approval being received approximately 5 years after the initiation of the first Phase I trial.

Sorafenib

PURPOSE OF REVIEW

Sorafenib is an oral, multikinase inhibitor that was recently approved for use in metastatic renal cancer. It is currently undergoing investigation in locally advanced renal cancer and in other tumor types.

RECENT FINDINGS

Sorafenib was initially developed as an inhibitor of Raf kinase; however, it has broad spectrum activity against multiple tyrosine kinases, including angiogenic factors VEGFR and PDGFR. Common toxicities experienced with sorafenib include hypertension, hand-foot syndrome, rash, diarrhea and fatigue. Early clinical trials suggested that sorafenib acts as a cytostatic agent, as many patients experienced prolonged disease stabilization but insufficient tumor shrinkage to meet RECIST criteria for response. To assess whether sorafenib's growth inhibition translated into a clinical benefit, a phase II randomized discontinuation trial was designed. This trial demonstrated that sorafenib increased progression-free survival in patients with metastatic renal cell cancer; the phase II data were confirmed in a large international phase III trial.

SUMMARY

In this review, we will discuss the clinical development of sorafenib and its role in the treatment of renal cancer. Additionally, we will highlight critical methods of clinical trial design and biomarker development that contribute to the development of sorafenib.

The metastatic cascade in prostate cancer

Morbidity and mortality due to prostate cancer are mainly a result of prostate cancer metastases. After the initial neoplastic transformation of cells, the process of metastasis involves a series of sequential steps, which involve neoangiogenesis and lymphangiogenesis, loss of adhesion with migration away from the primary tumour and entry into the systemic vasculature or lymphatics. Metastatic growth in sites such as lymph nodes and bone marrow then involves the specific non-random homing of prostate cancer cells. An appreciation and understanding of this metastatic cascade in relation to prostate cancer is clinically important in order to stratify men with prostate cancer into prognostic groups. Moreover, it is crucial in the future development of therapies that can prevent metastases.

Novel targets in prostate cancer

Despite recent advances in the understanding of the cellular and molecular biology of prostate cancer, new options for the treatment of prostate cancer remain elusive. Targeted therapies have shown promising activities in many solid tumours and the growing number of targets and targeted agents is creating numerous opportunities for clinical research in advanced prostate cancer. At ASCO 2006 in Atlanta, a clinical science symposium on novel targets in prostate cancer was presented. It consisted of three abstracts, each of which was followed by a discussant who reviewed the work and placed it in the overall context of current approaches to treating prostate cancer. The three abstracts were a discussion of a new method for quantification of the androgen receptor; the impact of high-dose vitamin D plus chemotherapy on hormone refractory disease and on the risk of thromboembolic disease; and the paradoxical effects seen with the raf-kinase and vascular endothelial growth factor inhibitor sorafenib, which produced improvement in bone scans in the absence of any prostate-specific antigen responses.