Chemotherapy in hormone-refractory prostate cancer

Calcitriol in Combination Therapy for Prostate Cancer: Pharmacokinetic and Pharmacodynamic Interactions

Epidemiological studies indicate that vitamin D insufficiency could have an etiological role in prostate cancer. In addition, calcitriol, used in combination with currently available drugs, has the potential to potentiate their anticancer effects or act synergistically by inhibiting distinct mechanisms involved in prostate cancer growth. Clinical data have not yet provided sufficient evidence to demonstrate benefit of vitamin D due to the limited and underpowered studies that have been published to date. Here, we review the preclinical and clinical studies that describe the activity of calcitriol, applied either alone or in combination and assessed the mechanistic basis of pharmacodynamic and pharmacokinetic interactions with calcitriol. Important considerations for calcitriol use in combination therapy with respect to safety and clinical outcomes have been discussed. Many of these combinations have therapeutic potential for the treatment of several cancer types and it is anticipated that future clinical research will put emphasis on well‑designed clinical trials to establish efficacy.

Clinical perspectives: practical insights from clinical experience with cabazitaxel in Australia

Prostate cancer, and in particular the management options for patients with metastatic castration-resistant prostate cancer (mCRPC), remains an important health issue. The approval of cabazitaxel provides a new treatment option for patients who have progressed despite docetaxel therapy. Clinical use of cabazitaxel in mCRPC is based on the results of the TROPIC study, which demonstrated an approximately 30% reduction in the risk of death compared with mitoxantrone. In this paper, we draw on accumulating clinical experience with cabazitaxel in Australia to discuss important questions such as how to select the right patient for treatment and how to proactively prevent and manage toxicities associated with this therapy. Recognizing the need for a multidisciplinary approach to patient care, opinion and insight has been sought from medical oncologists, nurses and pharmacists. While lack of trial data means that a number of questions remain unanswered, local clinical experience has helped to guide treatment decisions and refine management protocols. Appropriate patient selection, careful ongoing monitoring and proactive management of adverse events will ensure optimal treatment of patients. Nurses in particular play an important role in educating patients and identifying patients at increased risk of developing adverse events with cabazitaxel. The oncology team must work together to educate patients; taking a proactive approach to issues such as adverse events will help maximize the clinical outcome of cabazitaxel treatment and impact positively on the patient's quality of life.

Survival analysis of patients with biochemical relapse after radical prostatectomy treated with androgen deprivation: Castration-resistance influential factors

INTRODUCTION

We evaluate the prognosis of patients with biochemical recurrence (BCR) treated with androgen deprivation therapy (ADT) and to determine the influential factors to castration resistance (CR) and death.

METHODS

From a series of 1310 patients with T1-T2 prostate cancer treated with radical prostatectomy between 1989 and 2012, 371 had BCR. Patients with lymph node involvement were excluded. We analyzed only the 159 treated with salvage ADT. At the end of the study, 77 (48%) had developed CR.

RESULTS

The median follow-up to CR was 9.2 years. The CR-resistant free survival (RFS) was 76 ± 3%, 62 ± 3% and 43 ± 9% in 5, 10 and 15 years, respectively. The RFS median time was 14 years. In the multivariate study, the prostate-specific antigen (PSA) doubling time (PSA-DT) was <6 months (p = 0.01) (hazard ratio [HR] 3; 95% confidence interval [CI] 1.4-6.8, p = 0.007); seminal vesicle involvement (HR 3.1; 95% CI 1.5-6.2, p = 0.01) and PSA velocity in ng/mL/year (HR 1.3; 95% CI 1.1-1.5, p = 0.002) with better cut-off points of 0.84 ng/mL/year (p = 0.04) (HR 4; 95% CI 1.7-9.4, p = 0.001) were influential variables. Specific survival (SS) at 5, 10 and 15 years since surgery was 96 ± 1, 85 ± 2 and 76 ± 4, respectively. The time of CR to death was 30 ± 6% at 5 years, with the median at 3.2 years. In the multivariate only Ki 67 (HR 1.04; 95% CI 1.005-1.08, p = 0.02) had an independent influence.

CONCLUSIONS

In BCR patients treated with ADT, the median to CR was 14 years. PSA-DT <6 months, PSA velocity (ng/mL/year) and seminal vesicle involvement were influential variables. From the CR, the median time to death was 3.2 years. Ki-67 marker was an independent influence.

Predictive factors for premature discontinuation of docetaxel-based systemic chemotherapy in men with castration-resistant prostate cancer

Purpose

The objective was to determine predictive factors for premature discontinuation of docetaxel-based systemic chemotherapy in men with castration-resistant prostate cancer (CRPC).

Materials and Methods

We retrospectively reviewed the medical records of men who were treated with docetaxel-based systemic chemotherapy for CRPC in a single institution between May 2005 and April 2010. After being screened, 30 patients fit the eligibility criteria for inclusion in this study. Group 1 included 12 patients who were treated with five or fewer cycles of docetaxel chemotherapy for CRPC, and group 2 included 18 patients who were treated with six or more cycles of docetaxel chemotherapy for CRPC. The treatment consisted of 5 mg prednisolone twice daily and 75 mg/m² docetaxel once every 3 weeks.

Results

The median age was 72 years, and the median Eastern Cooperative Oncology Group (ECOG) performance status was 0. The median baseline prostate-specific antigen (PSA) level was 33.8 ng/mL. The median cycle of docetaxel-based chemotherapy was 5.8. Of 30 patients, 13 patients (48.2%) had a decline in PSA of >50% from baseline; 3 of 22 patients (13.6%) with measurable disease had achieved partial response on imaging. No differences in age, ECOG performance status, hemoglobin, serum creatinine, or PSA response were observed between the two groups. Body mass index was significantly lower (p=0.034) in group 1 (21.8 kg/m²) than in group 2 (23.6 kg/m²). Group 1 included more patients with prior systemic chemotherapy (p=0.039), and group 1 had a shorter overall survival rate (p=0.039).

Conclusions

Premature discontinuation of docetaxel-based systemic chemotherapy is associated with lower body mass index and prior systemic chemotherapy. Premature discontinuation of docetaxel-based chemotherapy is associated with a shorter overall survival rate.

7-epi-nemorosone from Clusia rosea induces apoptosis, androgen receptor down-regulation and dysregulation of PSA levels in LNCaP prostate carcinoma cells

The aim of this work was to characterize the antitumoral activity of the plant compound 7-epi-nemorosone in prostate carcinoma cell lines. Prostate cancer is the most frequently diagnosed malignancy and the second-leading cause of cancer death in men. In spite of the current therapeutic options for this cancer entity, many patients die due to metastases in distant organs and acquired chemotherapy resistance. Thus, approaches to provide improvements in outcome and quality of life for such patients are urgently needed. Recently, the polyisoprenylated benzophenone 7-epi-nemorosone, originally collected by honeybees from Clusia rosea and Clusia grandiflora (Clusiaceae), has been described to be a potent antitumoral agent. Here, its activity in prostate carcinoma is reported. 7-epi-nemorosone was isolated from Caribbean propolis employing RP-HPLC techniques. Its cytotoxicity was assessed using the MTT proliferation assay in human androgen-dependent prostate carcinoma LNCaP cells including an MDR1(+) sub-line. No cross-resistance was detected. FACS-based cell cycle analysis revealed a significant increase in the sub-G0/G1, G1, and depletion in the S phase populations. A concomitant down-regulation of cyclins D1/D3 and CDK 4/6 in LNCaP cells was detected by Western blot. Annexin-V-FITC labeling and caspase-3 cleavage assays showed that 7-epi-nemorosone induced apoptotic events. Major signal transduction elements such as p38 MAPK and Akt/PKB as well as androgen receptor AR and PSA production were found to be down-regulated after exposure to the drug. ERK1/2 protein levels and phosphorylation status were down-regulated accompanied by up-regulation but inhibition of the activity of their immediate upstream kinases MEK1/2. Additionally, Akt/PKB enzymatic activity was effectively inhibited at a similar concentration as for MEK1/2. Here, we demonstrate for the first time that 7-epi-nemorosone exerts cytotoxicity in an androgen-dependent prostate carcinoma entity by targeting the MEK1/2 signal transducer.

The experience with cytotoxic chemotherapy in metastatic castration-resistant prostate cancer

This article reviews the initial experience with chemotherapy in metastatic castration-resistant prostate cancer (mCRPC) and outlines some of the ongoing clinical trials in this area. In addition, the authors outline current knowledge on outcomes of patients treated with taxane-based chemotherapy on retrospective analysis of randomized trials. These data are intended to provide physicians and patients with a general idea on the outcomes of men with mCRPC that may facilitate clinical decisions as well as the design and evaluation of clinical trials.

Epithelial-to-mesenchymal transition leads to docetaxel resistance in prostate cancer and is mediated by reduced expression of miR-200c and miR-205

Docetaxel is a standard chemotherapy for patients with metastatic prostate cancer. However, the response is rather limited and not all of the patients benefit from this treatment. To uncover key mechanisms of docetaxel insensitivity in prostate cancer, we have established docetaxel-resistant sublines. In this study, we report that docetaxel-resistant cells underwent an epithelial-to-mesenchymal transition during the selection process, leading to diminished E-cadherin levels and up-regulation of mesenchymal markers. Screening for key regulators of an epithelial phenotype revealed a significantly reduced expression of microRNA (miR)-200c and miR-205 in docetaxel-resistant cells. Transfection of either microRNA (miRNA) resulted in re-expression of E-cadherin. Functional assays confirmed reduced adhesive and increased invasive and migratory abilities. Furthermore, we detected an increased subpopulation with stem cell-like properties in resistant cells. Tissue microarray analysis revealed a reduced E-cadherin expression in tumors after neoadjuvant chemotherapy. Low E-cadherin levels could be linked to tumor relapse. The present study uncovers epithelial-to-mesenchymal transition as a hallmark of docetaxel resistance. Therefore, we suggest that this mechanism is at least in part responsible for chemotherapy failure, with implications for the development of novel therapeutics.

[Docetaxel-based systemic chemotherapy in elderly Korean men with castration-resistant prostate cancer]

INTRODUCTION

The aim of this study was to determine the efficacy and safety of docetaxel-based systemic chemotherapy in elderly patients with castration-resistant prostate cancer (CRPC).

MATERIAL AND METHODS

We retrospectively reviewed the clinical records of 36 patients with CRPC who were treated with docetaxel-based systemic chemotherapy at a single institution between May 2005 and April 2010. After screening, 30 patients met the eligibility criteria, and were included. Patients were placed into 2 groups: group 1 consisted of 9 patients aged <70 years, and group 2 consisted of 21 patients aged ≥70 years. The treatment consisted of prednisolone (5mg) twice daily and docetaxel (75 mg/m(2)) once every 3 weeks.

RESULTS

The median age was 72 years, and the median performance status was 0. The median baseline prostate specific antigen (PSA) was 33.8 ng/mL. The mean number of docetaxel chemotherapy cycles was 5.8. The PSA response rate was 48.2%, and the measurable disease response rate was 15.0%, and these rates did not differ between the two groups. The median time to PSA progression and median overall survival were 6 and 9 months, respectively. Five patients experienced grade 3 or higher neutropenia. The drug-related toxicity was not different between the two groups.

CONCLUSIONS

The response of elderly CRPC patients with good performance status to docetaxel-based systemic chemotherapy was similar to that of younger patients. Docetaxel-based systemic chemotherapy is generally tolerated in elderly patients with good performance status.

Current paradigms and evolving concepts in metastatic castration-resistant prostate cancer

Until recently, docetaxel-based therapy represented the only therapy shown to prolong survival in patients with metastatic castration-resistant prostate cancer (mCRPC). The past year and a half has been marked by unprecedented progress in treatments for this disease. Three positive phase III clinical trials have emerged, each evaluating agents (sipuleucel-T, cabazitaxel and abiraterone) with distinct mechanisms of action. Herein, the three pivotal trials are described alongside both past and current large phase III studies conducted in this mCRPC. The overall survival for patients with mCRPC treated in current clinical trials is considerably longer than noted in the past. We note that more recent trials with older agents have also shown improved survival and discuss potential non-therapeutic biases that influence this critical measure of outcome. The necessity for utilizing randomized trials when evaluating new therapeutics is emphasized given the changing prognosis in this mCRPC.

Targeting the prostate-specific membrane antigen for prostate cancer therapy

Prostate cancer remains a leading cause of death for men in Western civilization. Despite the effectiveness of surgical prostatectomy, radiotherapy and hormonal therapy, a significant proportion of patients progress to advanced metastatic disease for which there are currently no curative treatment options. Therefore, new therapeutic approaches need to be considered. The prostate-specific membrane antigen is a cell-surface glycoprotein that is highly and specifically expressed on prostate epithelial cells and strongly upregulated in prostate cancer at all stages. These characteristics make it an attractive target for antibody-based imaging and therapies and the first anti-prostate-specific membrane antigen agents have already entered clinical trials. The proposed strategies include targeted toxins and radiotherapeutics as well as immunotherapeutic agents and vaccines.

Evolution of Docetaxel-Based Therapy for Metastatic Castrate-Resistant Prostate Cancer

Until the publication of the SWOG 99–16 [1] and TAX 327 [2] trials in 2004, urologists understood that only purely palliative treatment was available for men with prostate cancer that was progressive despite androgen deprivation. However, following the establishment of docetaxel-based chemotherapy as an active treatment option for most men with metastatic disease, researchers have focused their questions on the optimal timing of treatment. Should docetaxel be considered after one or two lines of hormone manipulation? Is it necessary to wait until the patient has bone pain? In latter years, the research focus has widened to embrace second-line and even third-line chemotherapy for this patient group, raising the possibility of advanced prostate cancer being managed as a chronic condition. This article looks at the evolution of docetaxel-based chemotherapy in advanced prostate cancer, and considers the next likely developments.

Cytochrome 450 1B1 (CYP1B1) polymorphisms associated with response to docetaxel in Castration-Resistant Prostate Cancer (CRPC) patients

BACKGROUND

The selection of patients according to key genetic characteristics may help to tailor chemotherapy and optimize the treatment in Castration-Resistant Prostate Cancer (CRPC) patients. Functional polymorphisms within the cytochrome P450 1B1 (CYP1B1) gene have been associated with alterations in enzymatic expression and activity and may change sensitivity to the widely used docetaxel regimen.

METHODS

CYP1B1 genotyping was performed on blood samples of 60 CRPC patients treated with docetaxel, using TaqMan probes-based assays. Association between CYP1B1-142C>G (leading to the 48ArgGly transition), 4326C>G (432LeuVal), and 4390A>G (453AsnSer) polymorphisms and treatment response, progression-free-survival (PFS) and overall-survival (OS) was estimated using Pearson χ2 test, Kaplan-Meier curves and Log-rank test.

RESULTS

Patients carrying the CYP1B1-432ValVal genotype experienced a significantly lower response-rate (P = 0.014), shorter progression-free-survival (P = 0.032) and overall-survival (P < 0.001). Multivariate analyses and correction for multiple comparisons confirmed its prognostic significance for OS. No significant associations were found among other polymorphisms and both response and clinical outcome.

CONCLUSIONS

CYP1B1-4326C>G (432LeuVal) polymorphism emerged as possible predictive marker of response and clinical outcome to docetaxel in CRPC patients and may represent a potential new tool for treatment optimization. Larger prospective trials are warranted to validate these findings, which might be applied to the future practice of CRPC treatment.

Rh2 or its aglycone aPPD in combination with docetaxel for treatment of prostate cancer

BACKGROUND

Docetaxel is one of the few chemotherapeutic drugs that are considered highly effective when used to treat prostate cancer patients that have relapsed and/or metastatic disease, it is therefore reasonable to expect further improvements in treatment outcomes when it is combined with other therapeutic agents active in prostate cancer. This study assesses the combination of well tolerated and orally bioavailable formulations of ginsenoside Rh2 or its aglycone aPPD with docetaxel.

METHODS

The in vitro activity of Rh2, aPPD, and docetaxel was determined in four prostate cancer cell lines: PC-3, LNCaP, DU145, and C4-2. Combinations of Rh2 or aPPD with docetaxel were assessed using the constant ratio combination design. Combination Indices (CI) and Dose Reduction Indices (DRI) were subsequently estimated using Calcusyn. In vivo efficacy studies and Immunohistochemical analyses (PC-3 model) were also evaluated.

RESULTS

In PC-3, DU145 and C4-2 prostate cancer cells combinations of Rh2 or aPPD with docetaxel were predominantly additive or synergistic. Combinations of Rh2 + docetaxel and aPPD + docetaxel caused established PC-3 tumors to regress from their initial size by 15% and 27%, respectively. Tumor cell proliferation rate (measured by Ki-67 positive cells) was significantly lower for combinations of Rh2 + docetaxel and aPPD + docetaxel, compared to animals treated with docetaxel alone.

CONCLUSIONS

Rh2 and aPPD can be combined with docetaxel to yield additive or synergistic activity in vitro and in vivo. Pending further assessment of toxicity and pharmacodynamic behavior, this study supports testing of combinations of ginsenoside Rh2 or its aglycone aPPD with docetaxel in a clinical setting.

Optimization of an albumin-binding prodrug of Doxorubicin that is cleaved by prostate-specific antigen

We have developed a novel albumin-binding prodrug of doxorubicin that incorporates p-aminobenzyloxycarbonyl (PABC) as a 1,6 self-immolative spacer in addition to the heptapeptide, Arg-Ser-Ser-Tyr-Tyr-Ser-Leu, as a substrate for the prostate-specific antigen (PSA) that is overexpressed in prostate carcinoma and represents a molecular target for selectively releasing an anticancer agent from a prodrug formulation. The prodrug exhibited good water solubility and was bound rapidly to the cysteine-34 position of human serum albumin. Incubation studies with PSA demonstrated that the albumin-bound form of the prodrug was cleaved rapidly at the P1-P1' scissile bond, releasing H-Ser-Leu-PABC-DOXO, which was further degraded to release doxorubicin as a final cleavage product within a few hours in prostate tumor tissue homogenates as well as in PSA-positive LNCaP LN cell lysates. Moreover, our prodrug exhibited antiproliferative activity in a low micromolar range against a PSA-expressing prostate cancer cell line (LNCaP).

Efficacy and safety of docetaxel plus prednisolone chemotherapy for metastatic hormone-refractory prostate adenocarcinoma: single institutional study in Korea

PURPOSE

To assess the efficacy and safety of treating Korean patients with metastatic hormone-refractory prostate cancer (HRPC) using docetaxel plus prednisolone chemotherapy.

MATERIALS AND METHODS

This was a retrospective cohort study performed in 98 patients with metastatic HRPC between October 2003 and April 2008. After screening, 72 patients fit the eligibility criteria for inclusion in this study. Treatment consisted of 5 mg prednisolone twice daily and 75 mg/m² docetaxel once every 3 weeks.

RESULTS

Patient demographic characteristics included: median age 67 years (range, 51~86), median ECOG performance status 1 (0~2), Gleason score ≥8 in 61 patients (86%), and median serum PSA 45.5 ng/mL (range, 3.7~2,420.0). A total of 405 cycles of treatment were administered with a median 6 cycles (range, 1~20) per patient. The median docetaxel dose-intensity was 24.4 mg/m(2)/week (range, 17.5~25.6). A PSA response was seen in 51% of 63 evaluable patients at 12 weeks and maximal PSA decline ≥50% in 59% of 70 evaluable patients. Tumor response was evaluated in 13 patients, 4 patients achieved PR, and 5 patients had SD with a response rate of 31%. With a median follow-up duration of 23.1 months (95%CI, 16.7~29.5), the median time to PSA progression was 5.1 months (95%CI, 4.5~5.8) and median overall survival was 22.8 months (95%CI, 16.6~29.1). Nine (13%) patients experienced grade 3 or higher febrile neutropenia.

CONCLUSION

This chemotherapy regimen (docetaxel every 3 weeks plus prednisolone daily) demonstrated a strong response in Korean patients with metastatic HRPC, while the toxicity profile was manageable and similar to that observed in Western patients.

Combination chemotherapy with weekly paclitaxel or docetaxel, carboplatin, and estramustine for hormone-refractory prostate cancer

Paclitaxel (PTX) and docetaxel (DTX) have been reported to be effective for treating hormone-refractory prostate cancer (HRPC). The objective of this study was to examine the efficacy of weekly DTX (PTX)-based chemotherapy and compare weekly DTX-based chemotherapy with triweekly (once every 3 weeks) DTX-based chemotherapy. We performed a combination chemotherapy on a weekly cycle with an i.v. PTX 100 mg/m(2) or i.v. DTX 30 mg/m(2) (days 1, 8, 15, and 22), i.v. carboplatin (CBDCA) (day 1, area under the plasma concentration time curve = 6), and oral estramustine phosphate 10 mg/kg daily for 10 HRPC patients. In addition, we investigated the patient characteristics and treatment efficacy and toxicity. Among all cases, serum prostate-specific antigen (PSA) decreased by 50% or more in 90% of patients, by 75% or more in 70%, and 90% or more in 40% after chemotherapy. The effectiveness of weekly DTX-based chemotherapy was comparable with previous reports, and we showed no toxicity serious enough to require cancellation of chemotherapy. In conclusion, weekly DTX-based chemotherapy was no less effective and less toxic than triweekly DTX-based chemotherapy for HRPC patients and therefore can be useful as the first-line chemotherapy regimen for HRPC patients, especially the elderly or those with a poor performance status.

Anti-metastatic effects of liposomal gemcitabine in a human orthotopic LNCaP prostate cancer xenograft model

Fatal outcomes of prostate carcinoma (PCa) mostly result from metastatic spread rather than from primary tumor burden. Here, we monitored growth and metastatic spread of an orthotopic luciferase/GFP-expressing LNCaP PCa xenograft model in SCID mice by in vivo imaging and in vitro luciferase assay of tissues homogenates. Although the metastatic spread generally shows a significant correlation to primary tumor volumes, the susceptibility of various tissues to metastatic invasion was different in the number of affected animals as well as in absolute metastatic burden in the individual tissues. Using this xenograft model we showed that treatment with liposomal gemcitabine (GemLip) inhibited growth of the primary tumors (83.9 +/- 6.4%; P = 0.009) as well as metastatic burden in lymph nodes (95.6 +/- 24.0%; P = 0.047), lung (86.5 +/- 10.5%; P = 0.015), kidney (88.4 +/- 9.2%; P = 0.045) and stomach (79.5 +/- 6.6%; P = 0.036) already at very low efficient concentrations (8 mg/kg) as compared to conventional gemcitabine (360 mg/kg). Our data show that this orthotopic LNCaP xenograft PCa model seems to reflect the clinical situation characterized by the fact that at time of diagnosis, prostate neoplasms are biologically heterogeneous and thus, it is a useful model to investigate new anti-metastatic therapies.

Liposomal gemcitabine (GemLip)-efficient drug against hormone-refractory Du145 and PC-3 prostate cancer xenografts

BACKGROUND

Gemcitabine (Gemc) is an efficient chemotherapeutic drug in various cancer types (e.g., pancreas) but has only limited effects on hormone-refractory prostate cancer (HRPCa). Since HRPCa cells are highly sensitive to even low doses of Gemc in vitro, the lack of clinical effects might be due to rapid degradation of Gemc by deaminases combined with impaired accumulation in tumor tissue and PCa cells. Liposomal formulation (GemLip) is expected to protect the entrapped cytotoxic substance from enzymatic degradation and furthermore augment its accumulation within tumor tissues due to an enhanced permeability of the tumor vessels.

METHODS

Anti-tumoral and anti-metastatic activity of GemLip and Gemc were investigated in two luciferase-expressing, human hormone-refractory PC-3 and Du145 HRPCa xenograft models in immunodeficient mice. Tumor growth was monitored by in vivo luminescence imaging (orthotopic) or callipering (subcutaneous). Anti-metastatic effects of treatment were determined by in vitro luciferase assay of the tissues.

RESULTS

Tumor growth of subcutaneous Du145 xenografts was significantly inhibited only by GemLip (8 mg/kg: P = 0.014 and 6 mg/kg: P = 0.011) but not by conventional Gemc (360 mg/kg). In contrast, growth of orthotopic PC-3 xenografts was significantly inhibited by both, GemLip (P = 0.041) and Gemc (P = 0.002). The drugs furthermore strongly reduced spleen and liver metastases in this model.

CONCLUSIONS

As shown by the very low efficient concentration of GemLip, liposomal entrapment of Gemc greatly enhances its activity. GemLip has, even at very low doses, a significant anti-tumoral and anti-metastatic therapeutic effect in HRPCa xenografts in vivo and was beneficial even when the conventional Gemc failed.

Ionizing radiation induces prostate cancer neuroendocrine differentiation through interplay of CREB and ATF2: implications for disease progression

Radiation therapy is a first-line treatment for prostate cancer patients with localized tumors. Although some patients respond well to the treatment, approximately 10% of low-risk and up to 60% of high-risk prostate cancer patients experience recurrent tumors. However, the molecular mechanisms underlying tumor recurrence remain largely unknown. Here we show that fractionated ionizing radiation (IR) induces differentiation of LNCaP prostate cancer cells into neuroendocrine (NE)-like cells, which are known to be implicated in prostate cancer progression, androgen-independent growth, and poor prognosis. Further analyses revealed that two cyclic AMP-responsive element binding transcription factors, cyclic AMP-response element binding protein (CREB) and activating transcription factor 2 (ATF2), function as a transcriptional activator and a repressor, respectively, of NE-like differentiation and that IR induces NE-like differentiation by increasing the nuclear content of phospho-CREB and cytoplasmic accumulation of ATF2. Consistent with this notion, stable expression of a nonphosphorylatable CREB or a constitutively nuclear-localized ATF2 in LNCaP cells inhibits IR-induced NE-like differentiation. IR-induced NE-like morphologies are reversible, and three IR-resistant clones isolated from dedifferentiated cells have acquired the ability to proliferate and lost the NE-like cell properties. In addition, these three IR-resistant clones exhibit differential responses to IR- and androgen depletion-induced NE-like differentiation. However, they are all resistant to cell death induced by IR and the chemotherapeutic agent docetaxel and to androgen depletion-induced growth inhibition. These results suggest that radiation therapy-induced NE-like differentiation may represent a novel pathway by which prostate cancer cells survive the treatment and contribute to tumor recurrence.