The cytostatic and cytotoxic effects of oridonin (Rubescenin), a diterpenoid from Rabdosia rubescens, on tumor cells of different lineage

Rabdosia rubescens is a herbal medicine used to treat esophageal cancer in China. In this study, the sesquiterpene oridonin, an isoprenoid, was isolated from Rabdosia rubescens. Mass spectroscopy and carbon 13 NMR spectroscopy were used to identify the structure of the purified compound. It was then evaluated for biological activity against human cell lines derived from prostate (DU-145, LNCaP), breast (MCF-7), and ovarian (A2780 and PTX10) cancers. Oridonin exhibited anti-proliferative activity toward all cancer cell lines tested, with an IC50 estimated by the MTT cell viability assay ranging from 5.8+/-2.3 to 11.72+/-4.8 microM. Flow cytometric analysis demonstrated that oridonin induced a G1 phase arrest in androgen receptor-positive LNCaP cells containing wt p53, while it blocked the cell cycle at G2 and M phases in androgen receptor-negative DU-145 cells with mutated p53; the arrest in M was verified by examination of cell morphology and by the increased frequency of cells with Ser-10 phosphorylated histone H3. The increased incidence of apoptosis, identified by characteristic changes in cell morphology, was seen in tumor lines treated with oridonin. Notably, at concentrations that induced apoptosis among tumor cells, oridonin failed to induce apoptosis in cultures of normal human fibroblasts. Western blot analysis was used to determine the protein expression of cancer suppressor genes, p53 (wt) and Bax, and the proto-oncogene, Bcl-2 in LNCaP cells following treatment with oridonin. Oridonin up-regulated p53 and Bax and down-regulated Bcl-2 expression in a dose-dependent manner. To further explore the possible interaction between oridonin and DNA, its absorption spectrum was measured in the presence and absence of double stranded (ds) DNA. Spectral shifts and an increase in absorption band intensity were observed indicating interaction of oridonin with DNA bases. The nature of the binding is not clear at present though no evidence of histone H2AX phosphorylation on Ser-139 was apparent in DU-145 cells treated with oridonin that would indicate the induction of ds DNA breaks. In conclusion, oridonin inhibits cancer cell growth in a cell cycle specific manner and shifts the balance between pro- and anti-apoptotic proteins in favor of apoptosis. The present data suggest that further studies are warranted to assess the potential of oridonin in cancer prevention and/or treatment.     

Cell cycle arrest and apoptotic induction in LNCaP cells by MCS-C2, novel cyclin-dependent kinase inhibitor, through p53/p21WAF1/CIP1 pathway

The purpose of the present study was to investigate the mechanisms involved in the antiproliferative and apoptotic effects of MCS-C2, a novel analog of the pyrrolo[2,3-d]pyrimidine nucleoside toyocamycin and sangivamycin, in human prostate cancer LNCaP cells. MCS-C2, a selective inhibitor of cyclin-dependent kinase, was found to inhibit cell growth in a time- and dose-dependent manner, and inhibit cell cycle progression by inducing the arrest of the G1 phase and apoptosis in LNCaP cells. When treated with 3 microM MCS-C2, inhibited proliferation associated with apoptotic induction was found in the LNCaP cells in a concentration and time-dependent manner, and nuclear DAPI staining revealed the typical nuclear features of apoptosis. Furthermore, MCS-C2 induced cell cycle arrest in the G1 phase through the upregulated phosphorylation of the p53 protein at Ser-15 and activation of its downstream target gene p21WAF1/CIP1. Accordingly, these results suggest that MCS-C2 inhibits the proliferation of LNCaP cells by way of G1-phase arrest and apoptosis in association with the regulation of multiple molecules in the cell cycle progression.     

Oridonin enhances antitumor activity of gemcitabine in pancreatic cancer through MAPK-p38 signaling pathway

Gemcitabine is currently the best treatment available for pancreatic cancer (PaCa); however, patients with the disease develop resistance to the drug over time. Agents that can either enhance the effects of gemcitabine or overcome chemoresistance to the drug are required for the treatment of PaCa. Oridonin is one such agent which is safe and multitargeted, and has been linked with the suppression of survival, proliferation, invasion and angiogenesis of cancer. In this study, we investigated whether oridonin could sensitize PaCa to gemcitabine in vitro and in vivo. In vitro, oridonin inhibited the proliferation of the PaCa cell line, BxPC-3, potentiated the apoptosis induced by gemcitabine, induced G1 cell cycle arrest and activated p38 and p53; these results were significant when oridonin was combined with gemcitabine. In vivo, we found that oridonin significantly suppressed tumor growth and this effect was further enhanced by gemcitabine (P<0.05). Tumors from nude mice injected with BxPC-3 PaCa cells and treated with a combination of oridonin and gemcitabine showed a significant upregulation in p38 and p53 activation (P<0.05 vs. control, P<0.05 vs. gemcitabine or oridonin alone). Taken together, our results demonstrate that oridonin can potentiate the effects of gemcitabine in PaCa through the mitogen-activated protein kinase (MAPK)-p38 signaling pathway, which is dependent on p53 activation.     

Dual effects of glutathione-S-transferase pi on As2O3 action in prostate cancer cells: enhancement of growth inhibition and inhibition of apoptosis

To determine the effects of glutathione-S-transferase pi (GSTpi) on the actions of As2O3, As2O3-induced growth inhibition and apoptosis was studied in three prostate cancer cell lines: DU-145, PC-3 and LNCaP cells. As2O3 inhibited cell proliferation of DU-145 and PC-3 cells (both cells express GSTpi), but not of LNCaP cells (which lack GSTpi expression) at concentrations below 1 microM. LNCaP cells stably transfected and expressed GSTpi (LNCaP/GSTpi) became sensitive to As2O3 growth inhibition. As2O3 arrested cell growth of DU-145, PC-3 and LNCaP/GSTpi cells in the G2/M phase of the cell cycle at low concentrations (<2 microM), but did not induce apoptosis. At higher concentrations (10-20 microM), As2O3 induced apoptosis in LNCaP cells, but not in DU-145 or PC-3 cells. The apoptosis induction due to As2O3 treatment of LNCaP cell correlated with the activation of JNK and p38 and induction of p53 protein. LNCaP/GSTpi cells became insensitive to As2O3-induced apoptosis with reduced JNK activition. These data indicate that GSTpi increases growth inhibition due to As2O3 treatment and prevents As2O3-induced apoptosis in prostate cancer cells. Therefore, it appears that As2O3 inhibits cell growth and induces apoptosis through different mechanisms.     

Saw Palmetto induces growth arrest and apoptosis of androgen-dependent prostate cancer LNCaP cells via inactivation of STAT 3 and androgen receptor signaling

PC-SPES is an eight-herb mixture that has an activity against prostate cancer. Recently, we purified Saw Palmetto (Serenoa repens) from PC-SPES and found that Saw Palmetto induced growth arrest of prostate cancer LNCaP, DU145, and PC3 cells with ED50s of approximately 2.0, 2.6, and 3.3 microl/ml, respectively, as measured by mitochondrial-dependent conversion of the the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. Saw Palmetto induced apoptosis of LNCaP cells in a time- and dose-dependent manner as measured by TUNEL assays. Also, Saw Palmetto increased the expression of p21waf1 and p53 protein in LNCaP cells. In addition, we found that Saw Palmetto down-regulated DHT- or IL-6-induced expression of prostate specific antigen in conjunction with down-regulation of the level of androgen receptor in the nucleus as measured by Western blot analysis. Moreover, Saw Palmetto down-regulated the IL-6-induced level of the phosphorylated form of STAT 3 in LNCaP cells. Furthermore, Saw Palmetto inhibited the growth of LNCaP cells present as tumor xenografts in BALB/c nude mice without adverse effect. These results indicate that Saw Palmetto might be useful for the treatment of individuals with prostate cancer.     

Oridonin-induced apoptosis in SW620 human colorectal adenocarcinoma cells

Oridonin, a diterpenoid isolated from Rabdosia rubescens (Hemsl.) Hara, inhibited the growth of human tumor cell lines SW620 (colon), MCF-7 (breast) and K562 (bone marrow), and induced significant levels of apoptosis in SW620. Morphological changes indicative of cell apoptosis were observed after the cells were exposed to oridonin for 24 h. Growth inhibition was associated with G1 phase arrest, and with time- and dose-dependent increases in caspase-3 activity. We therefore conclude that oridonin inhibits the proliferation of SW620 cells by induction of apoptosis via the activation of caspase-3. Our data suggest that oridonin may have significant potential as an anti-colorectal adenocarcinoma agent.     

Antiproliferative effects of AVN944, a novel inosine 5-monophosphate dehydrogenase inhibitor, in prostate cancer cells

Inosine 5-monophosphate dehydrogenase II, a key enzyme in the de novo synthesis of purine nucleotides, is expressed in prostate tumors and prostate cancer cells. AVN944 is a new, specific, noncompetitive IMPDH inhibitor. In this study, we investigated the effects of IMPDH inhibitor AVN944 on LNCaP, CWR22Rv1, DU145 and PC-3 human prostate cancer cells. AVN944 inhibited proliferation of these 4 prostate cancer cell lines and was associated with cell cycle G1 arrest of LNCaP cells and S-phase block of androgen-independent CWR22Rv1, DU145 and PC-3 cells. AVN944 induced caspase-dependentand caspase-independent cell death in LNCaP, CWR22Rv1, and DU145 cells. AVN944 induced expression of p53-target proteins Bok, Bax and Noxa in androgen-responsive cell lines and suppressed expression of survivin in prostate cancer cells regardless of their androgen sensitivity. AVN944 also induced differentiation of androgen-independent prostate cancer cells as indicated by morphological changes and increased expression of genes coding for prostasomal proteins, keratins and other proteins, including tumor suppressor genes MIG-6 and NDRG1. AVN944-differentiated androgen-independent DU145 and PC-3 cells are sensitized to TRAIL-induced apoptosis as demonstrated by induction of caspases and PARP cleavage. In summary, AVN944 inhibited the growth of human prostate cancer cells by inducing cell cycle arrest, cell death as well as differentiation. AVN944 is a novel, promising therapeutic agent that might be combined with other agents for treatment of human prostate cancer.     

Effects of PC-SPES on proliferation and expression of AR/PSA in androgen-responsive LNCaP cells are independent of estradiol

Previous studies have suggested that the clinical efficacy of PC-SPES, a dietary supplement used frequently by men diagnosed with androgen-dependent (AD) or androgen-independent (AI) prostate cancer (CaP), is mechanistically attributed to estrogenic components present in the herbal mixture. To test this hypothesis, we compared estradiol (1 nM), potentially an active principle in PC-SPES, with PC-SPES (using an amount equivalent to 1 nM estradiol) on cell proliferation, induction of apoptosis, and regulation of prostate specific genes, PSA and AR, in androgen-responsive LNCaP cells. Cells cultured in steroid-proficient (FBS) or-deficient (CS-FBS) media to simulate hormonal status pre- and post-castration in vivo, were incubated with estradiol or PC-SPES. Proliferation was reduced in PC-SPES treated cells cultured in media supplemented with FBS or CS-FBS; in contrast, addition of estradiol had no effect on proliferation in FBS cultures, and elicited a 45% growth increase in CS-PBS-supplemented cultures. The differential proliferative response of LNCaP cells to PC-SPES vs. estradiol was also supported by changes in PCNA expression, cell viability, cell cycle phase distribution, and induction of apoptosis. Estradiol elicited time-dependent increases in secreted PSA, whereas PC-SPES suppressed PSA secretion, in both culture conditions. In FBS cultures, PC-SPES lowered intracellular AR and PSA by 61% and 17%, respectively, while estradiol increased intracellular PSA, in parallel with a 42% decrease in AR expression. In comparison with cells maintained with CS-FBS, estradiol induced substantial increases in both intracellular PSA and AR, whereas PC-SPES resulted in a smaller increase in intracellular PSA without affecting the expression of AR. These studies show that the antiproliferative and gene modulatory effects of PC-SPES in androgen-dependent human prostate cancer cells are mechanistically and functionally distinct from effects attributable to estradiol.     

Inhibition of phosphatidylinositol-3-kinase causes cell death through a protein kinase B (PKB)-dependent mechanism and growth arrest through a PKB-independent mechanism

PURPOSE: To identify whether inhibition of phosphatidylinositol-3-kinase (PI3K) causes apoptosis through inhibition of protein kinase B (PKB), implicating PKB as an important therapeutic target in prostate cancer. METHODS AND MATERIALS: After treatment with the PI3K inhibitor, LY294002, proliferation and apoptosis of the prostate cancer cell line, LNCaP, were measured by cell cycle analysis and cleavage of poly (ADP-ribose) polymerase. To test the hypothesis that inhibition of PKB is responsible for the LY294002-induced apoptosis, LNCaP cells expressing a constitutively active form of PKB were generated. RESULTS: Treatment of LNCaP cells with the PI3K inhibitor, LY294002, caused inactivation of PKB, growth arrest, and apoptosis. LY294002-induced apoptosis was increased in the absence of serum. The G1 growth arrest was associated with an increase in p27(kip1) expression. Cells expressing constitutively active PKB were protected from apoptosis induced by LY294002, but not from the G1 growth arrest induced by PI3K inhibition. CONCLUSION: These data suggest that PKB activity regulates apoptosis, but not G1 arrest, and identify PKB as a potential critical target for cancer therapy. Targeted therapy against kinases might complement more conventional therapies, including androgen suppression for prostate cancer.     

2-Methoxyestradiol and paclitaxel have similar effects on the cell cycle and induction of apoptosis in prostate cancer cells

2-Methoxyestradiol (2-ME) is an endogenous metabolite of estradiol with promise for cancer chemotherapy, including advanced prostate cancer. We have focused on events related to cell cycle arrest (G1 and G2/M) and induction of apoptosis in human prostate cancer cells. Treatment with 2-ME increased cyclin B1 protein and its associated kinase activity followed by later inhibition of cyclin A-dependent kinase activity and induction of apoptosis. Similar results were obtained with paclitaxel (taxol), a clinically relevant agent used to treat advanced prostate cancer. Cyclin-dependent kinase inhibitors prevented 2-ME and paclitaxel-mediated increase in cyclin B1-dependent kinase activity and blocked induction of apoptosis. Reduction of X-linked inhibitor of apoptosis (XIAP) protein by 2-ME and paclitaxel correlated with increased apoptosis. Lower doses of 2-ME and paclitaxel resulted in G1 (but not G2/M) cell cycle arrest in the p53 wild type LNCaP cell line, but with minimal induction of apoptosis. We suggest that 2-ME and paclitaxel-mediated induction of apoptosis in prostate cancer cells requires activation of cyclin B1-dependent kinase that arrests cells in G2/M and subsequently leads to the induction of apoptotic cell death.     

Induction of G1 arrest and apoptosis in androgen-dependent human prostate cancer by Kuguacin J,a triterpenoid from Momordica charantia leaf

In this study, we focused on the effects of a bitter melon (Momordica charantia) leaf extract (BMLE) and a purified component, Kuguacin J (KuJ), on androgen-dependent LNCaP human prostate cancer cells. Both treatments exerted growth inhibition through G1 arrest and induction of apoptosis. In addition, KuJ markedly decreased the levels of cyclins (D1 and E), cyclin-dependent kinases (Cdk2 and Cdk4) and proliferating cell nuclear antigen, and caused an increase in p21 and p27 levels. Its induction of apoptosis was accompanied by an increase in cleavage of caspase-3 and poly (ADP-ribose) polymerase, attributable to augment of Bax/Bcl-2 and Bad/Bcl-xL and reduction of survivin levels. BMLE and KuJ also reduced the expression of androgen receptor (AR), prostate-specific antigen (PSA) while induced P53 protein level. Down-regulation of p53 by RNA interference indicated that BMLE and KuJ inhibited cell growth partly through p53-dependent cell cycle arrest and apoptotic pathways. Both BMLE and KuJ caused less toxicity in a normal prostate cell line, PNT1A. Our results suggest that BMLE and a purified component, KuJ, from its diethyl ether fraction could be promising candidate new antineoplastic and chemopreventive agents for androgen-dependent prostate cancer and carcinogenesis.     

Effects of the herbal extract PC-SPES on microtubule dynamics and paclitaxel-mediated prostate tumor growth inhibition

BACKGROUND: PC-SPES is a botanical preparation shown to have efficacy in patients with androgen-dependent and androgen-independent prostate carcinoma. Several herbal constituents in PC-SPES inhibit tumor growth through cell cycle arrest and apoptosis, although the mechanisms of these activities are poorly defined. We sought to identify PC-SPES-induced changes in gene expression, specifically in those genes encoding cytoskeletal proteins that could be associated with PC-SPES-induced cytoxicity. METHODS: LNCaP prostate carcinoma cells were treated with PC-SPES, and changes in gene expression were determined by complementary DNA (cDNA) microarray hybridization and northern blot analyses. PC-SPES and paclitaxel, a microtubule-stabilizing drug, effects on microtubules were assessed by immunofluorescence of treated cells and by in vitro tubulin polymerization assays. In vivo effects of PC-SPES and paclitaxel were assessed using CWR22R androgen-independent prostate cancer xenografts. All statistical tests were two-sided. RESULTS: PC-SPES treatment of LNCaP cells for 24 hours altered the expression of 17 cytoskeletal genes. mRNA levels of alpha-tubulin decreased sevenfold. Although paclitaxel stabilized and PC-SPES treatment disrupted microtubule architecture in LNCaP cells, the combination of both agents had an intermediate effect. PC-SPES inhibited tubulin polymerization in vitro, even in the presence of paclitaxel. Compared with tumors in control mice (mean tumor volume = 2983 mm(3), 95% confidence interval [CI] = 2380 to 3586 mm(3)), tumors were statistically significantly smaller in mice that received PC-SPES (mean tumor volume = 2018 mm(3), 95% CI = 1450 to 2568 mm(3); P =.028), paclitaxel (mean tumor volume = 1340 mm(3), 95% CI = 697 to 1983 mm(3); P<.001), or the combination of PC-SPES and paclitaxel (mean tumor volume = 1955 mm(3), 95% CI = 1260 to 2650 mm(3); P =.034). CONCLUSION: PC-SPES may interfere with microtubule polymerization. This activity has implications for the clinical management of patients with advanced prostate cancer who may be taking PC-SPES concurrently with microtubule-modulating chemotherapeutic agents, such as paclitaxel.     

Resistance to growth inhibitory and apoptotic effects of phorbol ester and UCN-01 in aggressive cancer cell lines

7-Hydroxystaurosporine (UCN-01), a non-selective inhibitor of protein kinase C (PKC), and phorbol ester (PMA), a PKC activator, are undergoing clinical evaluations. We investigated the effects of UCN-01 and PMA on a panel of prostate cancer cell lines. While PMA induced p21WAF1/CIP1 and arrest growth of LNCaP cancer cells (IC50 = 0.5-1 nM), aggressive cancer cell lines (DU145, PC3, and PC3M) were resistant to PMA (IC50 >5000 nM). Low concentrations (25-50 nM) of UCN-01 abrogated PMA-induced p21 and growth arrest in LNCaP cells. These low doses of UCN-01 however did not inhibit proliferation of any prostate cancer cell line. PMA-sensitive LNCaP cells were resistant to clinically relevant concentrations of UCN-01 (IC50 = 1.2 microM), but UCN-01 inhibited growth of DU145 and PC3/3M with an IC50 of 200-400 nM. For comparison, PMA-sensitive HL60 leukemia cells were sensitive to UCN-01 due to rapid apoptosis caused by UCN-01. In PMA-resistant prostate cancer cells, UCN-01 downregulated cyclin D1, induced p21, caused morphological differentiation, and G1-phase arrest leading to slow cell death without caspase activation. Importantly, normal prostate epithelial cells (PrEC) were very sensitive to both PMA (IC50 = 0.2 nM) and UCN-01. In PrEC, UCN-01 downregulated cyclin D1 and arrest growth with an IC50 less than 100 nM. We conclude that loss of sensitivity to either UCN-01 or PMA accompanies progression of prostate cancer.     

The iroquois homeobox gene 5 is regulated by 1,25-dihydroxyvitamin D3 in human prostate cancer and regulates apoptosis and the cell cycle in LNCaP prostate cancer cells.

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], the most active metabolite of vitamin D3, has significant antitumor activity in a broad range of preclinical models of cancer. In this study, we show that the Iroquois homeobox gene 5 (Irx5) is down-regulated by 1,25(OH)2D3 in human prostate cancer samples from patients randomly assigned to receive weekly high-dose 1,25(OH)2D3 or placebo before radical prostatectomy. Down-regulation of Irx5 by 1,25(OH)2D3 was also shown in the human androgen-sensitive prostate cancer cell line LNCaP and in estrogen-sensitive MCF-7 breast cancer cells. Knockdown of Irx5 by RNA interference showed a significant reduction in LNCaP cell viability, which was accompanied by an increase in p21 protein expression, G2-M arrest, and an increase in apoptosis. The induced apoptosis was partially mediated by p53, and p53 protein expression was increased as a result of Irx5 knockdown. Cell survival was similarly reduced by Irx5 knockdown in the colon cancer cell line HCT 116 and in MCF-7 breast cancer cells, each being derived from clinical tumor types that seem to be inhibited by 1,25(OH)2D3. Overexpression of Irx5 led to a reduction of p21 and p53 expression. This is the first report that Irx5 is regulated by 1,25(OH)2D3 in humans and the first report to show that Irx5 is involved in the regulation of both the cell cycle and apoptosis in human prostate cancer cells. Irx5 may be a promising new therapeutic target in cancer treatment.     

Growth of human prostate cancer cells is significantly suppressed in vitro with sodium butyrate through apoptosis

Histone deacetylase inhibitors (HDACis) have shown significant antiproliferative and apoptotic properties in various types of cancer cells, including prostate cancer cells, and are therefore being evaluated as a treatment modality. However, the mechanism by which sodium butyrate (SB) induces apoptosis is not completely understood. We focused on SB which exists in the intestine and is therefore expected to have less adverse effects. In this study, three prostate cancer cell lines (LNCaP, DU145 and PC-3) were treated in vitro with different concentrations of SB. Cell proliferation was studied by the XTT assay; cell cycle analysis and induction of apoptosis were studied by laser scanning cytometry. Western blot analysis was used to study p21, p27, CDK2, CDK4, CDK6, caspase-3, caspase-7, Fas, FADD, TRADD, Bcl-2 and Bax protein expression. SB inhibited cell growth and induced apoptosis in a concentration-dependent manner in human prostate cancer cells (LNCaP, DU145 and PC-3). Western blot analysis showed dose-dependent increases of p21 levels in DU145 and PC-3 cells, and dose-dependent decreases of CDK2, CDK4, CDK6 and procaspase-3 protein levels in all three prostate cancer cell lines. Bcl-xL was significantly down-regulated in DU145 cells, and Bcl-2 was significantly down-regulated in PC-3 and LNCaP cells. No significant changes were observed in procaspase-7, TRADD and Bax expression, although slight decreases in Fas and FADD expression were seen in all three prostate cancer cell lines. Analysis of cell morphology using laser scanning microscopy detected condensed and fragmented nuclei. In conclusion, SB induces G1 and G2 arrest by increasing p21 expression resulting in CDK2, CDK4 and CDK6 down-regulation. SB potently induced apoptosis, which was accompanied by DNA fragmentation, down-regulated Bcl-2 in LNCaP and PC-3 cells, Bcl-xL in DU145 cells, and down-regulated procaspase-3, but not procaspase-7, in these human prostate cancer cell lines. These results suggest that SB may serve as a new modality for the treatment of hormone refractory prostate cancer.     

VMY-1-103, a dansylated analog of purvalanol B,induces caspase-3-dependent apoptosis in LNCaP prostate cancer cells

The 2,6,9-trisubstituted purine group of cyclin dependent kinase inhibitors have the potential to be clinically relevant inhibitors of cancer cell proliferation. We have recently designed and synthesized a novel dansylated analog of purvalanol B, termed VMY-1-103, that inhibited cell cycle progression in breast cancer cell lines more effectively than did purvalanol B and allowed for uptake analyses by fluorescence microscopy.ErbB-2 plays an important role in the regulation of signal transduction cascades in a number of epithelial tumors, including prostate cancer (PCa). Our previous studies demonstrated that transgenic expression of activated ErbB-2 in the mouse prostate initiated PCa and either the overexpression of ErbB-2 or the addition of the ErbB-2/ErbB-3 ligand, heregulin (HRG), induced cell cycle progression in the androgen-responsive prostate cancer cell line, LNCaP.In the present study, we tested the efficacy of VMY-1-103 in inhibiting HRG-induced cell proliferation in LNCaP prostate cancer cells. At concentrations as low as 1 µM, VMY-1-103 increased both the proportion of cells in G₁ and p21^CIP1 protein levels. At higher concentrations (5 µM or 10 µM), VMY-1-103 induced apoptosis via decreased mitochondrial membrane polarity and induction of p53 phosphorylation, caspase-3 activity and PARP cleavage. Treatment with 10 µM Purvalanol B failed to either influence proliferation or induce apoptosis.Our results demonstrate that VMY-1-103 was more effective in inducing apoptosis in PCa cells than its parent compound, purvalanol B, and support the testing of VMY-1-103 as a potential small molecule inhibitor of prostate cancer in vivo.Key words: prostate cancer, apoptosis, cell cycle, CDK inhibitor, p53