Phase II, Open-Label Study of Ciltacabtagene Autoleucel, an Anti-B-Cell Maturation Antigen Chimeric Antigen Receptor-T-Cell Therapy, in Chinese Patients With Relapsed/Refractory Multiple Myeloma (CARTIFAN-1)

PURPOSE

CARTIFAN-1 aimed to evaluate the efficacy and safety of ciltacabtagene autoleucel (cilta-cel), a B-cell maturation antigen-targeting chimeric antigen receptor T-cell therapy, in Chinese patients with relapsed/refractory multiple myeloma (RRMM).

METHODS

This pivotal phase II, open-label study (ClinicalTrials.gov identifier: NCT03758417), conducted across eight sites in China, enrolled adult patients with RRMM who had received ≥ 3 lines of prior therapy, including a proteasome inhibitor and immunomodulatory drug. Patients received a single infusion of cilta-cel (target dose 0.75 × 10⁶ chimeric antigen receptor-positive viable T cells/kg). The primary end point was overall response rate. Secondary end points included progression-free survival (PFS), overall survival (OS), and incidence and severity of adverse events (AEs).

RESULTS

As of the clinical cutoff of July 19, 2021, 48 patients received a cilta-cel infusion. At an 18-month median follow-up, the overall response rate was 89.6% (95% CI, 77.3 to 96.5), with a median time to first response of approximately 1 month; 77.1% of patients (95% CI, 62.7 to 88.0) achieved complete response or better. Medians for duration of response, PFS, and OS were not reached. The 18-month PFS and OS rates were 66.8% (95% CI, 49.4 to 79.4) and 78.7% (95% CI, 64.0 to 88.0), respectively. Hematologic AEs were common, including anemia (100%), neutropenia (97.9%), lymphopenia (95.8%), and thrombocytopenia (87.5%). Cytokine release syndrome occurred in 97.9% of patients (35.4% grade 3/4); the median time to onset was 7 days, and the median duration was 5 days. Infections occurred in 85.4% of patients (37.5% grade 3/4). Ten deaths occurred after cilta-cel infusion, eight of which were due to treatment-related AEs.

CONCLUSION

These data demonstrate a favorable risk-benefit profile for a single infusion of cilta-cel, resulting in early, deep, and durable responses in heavily pretreated patients with RRMM in China.

Ciltacabtagene autoleucel, a B-cell maturation antigen-directed chimeric antigen receptor T-cell therapy in patients with relapsed or refractory multiple myeloma (CARTITUDE-1): a phase 1b/2 open-label study

BACKGROUND

CARTITUDE-1 aimed to assess the safety and clinical activity of ciltacabtagene autoleucel (cilta-cel), a chimeric antigen receptor T-cell therapy with two B-cell maturation antigen-targeting single-domain antibodies, in patients with relapsed or refractory multiple myeloma with poor prognosis.

METHODS

This single-arm, open-label, phase 1b/2 study done at 16 centres in the USA enrolled patients aged 18 years or older with a diagnosis of multiple myeloma and an Eastern Cooperative Oncology Group performance status score of 0 or 1, who received 3 or more previous lines of therapy or were double-refractory to a proteasome inhibitor and an immunomodulatory drug, and had received a proteasome inhibitor, immunomodulatory drug, and anti-CD38 antibody. A single cilta-cel infusion (target dose 0·75 × 10⁶ CAR-positive viable T cells per kg) was administered 5-7 days after start of lymphodepletion. The primary endpoints were safety and confirmation of the recommended phase 2 dose (phase 1b), and overall response rate (phase 2) in all patients who received treatment. Key secondary endpoints were duration of response and progression-free survival. This trial is registered with ClinicalTrials.gov, NCT03548207.

FINDINGS

Between July 16, 2018, and Oct 7, 2019, 113 patients were enrolled. 97 patients (29 in phase 1b and 68 in phase 2) received a cilta-cel infusion at the recommended phase 2 dose of 0·75 × 10⁶ CAR-positive viable T cells per kg. As of the Sept 1, 2020 clinical cutoff, median follow-up was 12·4 months (IQR 10·6-15·2). 97 patients with a median of six previous therapies received cilta-cel. Overall response rate was 97% (95% CI 91·2-99·4; 94 of 97 patients); 65 (67%) achieved stringent complete response; time to first response was 1 month (IQR 0·9-1·0). Responses deepened over time. Median duration of response was not reached (95% CI 15·9-not estimable), neither was progression-free survival (16·8-not estimable). The 12-month progression-free rate was 77% (95% CI 66·0-84·3) and overall survival rate was 89% (80·2-93·5). Haematological adverse events were common; grade 3-4 haematological adverse events were neutropenia (92 [95%] of 97 patients), anaemia (66 [68%]), leukopenia (59 [61%]), thrombocytopenia (58 [60%]), and lymphopenia (48 [50%]). Cytokine release syndrome occurred in 92 (95%) of 97 patients (4% were grade 3 or 4); with median time to onset of 7·0 days (IQR 5-8) and median duration of 4·0 days (IQR 3-6). Cytokine release syndrome resolved in all except one with grade 5 cytokine release syndrome and haemophagocytic lymphohistiocytosis. CAR T-cell neurotoxicity occurred in 20 (21%) patients (9% were grade 3 or 4). 14 deaths occurred in the study; six due to treatment-related adverse events, five due to progressive disease, and three due to treatment-unrelated adverse events.

INTERPRETATION

A single cilta-cel infusion at the target dose of 0·75 × 10⁶ CAR-positive viable T cells per kg led to early, deep, and durable responses in heavily pretreated patients with multiple myeloma with a manageable safety profile. The data from this study formed the basis for recent regulatory submissions.

FUNDING

Janssen Research & Development and Legend Biotech.

Randomized Phase III Trial of Ibrutinib and Rituximab Plus Cyclophosphamide, Doxorubicin, Vincristine, and Prednisone in Non-Germinal Center B-Cell Diffuse Large B-Cell Lymphoma

PURPOSE

Ibrutinib has shown activity in non-germinal center B-cell diffuse large B-cell lymphoma (DLBCL). This double-blind phase III study evaluated ibrutinib and rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) in untreated non-germinal center B-cell DLBCL.

PATIENTS AND METHODS

Patients were randomly assigned at a one-to-one ratio to ibrutinib (560 mg per day orally) plus R-CHOP or placebo plus R-CHOP. The primary end point was event-free survival (EFS) in the intent-to-treat (ITT) population and the activated B-cell (ABC) DLBCL subgroup. Secondary end points included progression-free survival (PFS), overall survival (OS), and safety.

RESULTS

A total of 838 patients were randomly assigned to ibrutinib plus R-CHOP (n = 419) or placebo plus R-CHOP (n = 419). Median age was 62.0 years; 75.9% of evaluable patients had ABC subtype disease, and baseline characteristics were balanced. Ibrutinib plus R-CHOP did not improve EFS in the ITT (hazard ratio [HR], 0.934) or ABC (HR, 0.949) population. A preplanned analysis showed a significant interaction between treatment and age. In patients age younger than 60 years, ibrutinib plus R-CHOP improved EFS (HR, 0.579), PFS (HR, 0.556), and OS (HR, 0.330) and slightly increased serious adverse events (35.7% v 28.6%), but the proportion of patients receiving at least six cycles of R-CHOP was similar between treatment arms (92.9% v 93.0%). In patients age 60 years or older, ibrutinib plus R-CHOP worsened EFS, PFS, and OS, increased serious adverse events (63.4% v 38.2%), and decreased the proportion of patients receiving at least six cycles of R-CHOP (73.7% v 88.8%).

CONCLUSION

The study did not meet its primary end point in the ITT or ABC population. However, in patients age younger than 60 years, ibrutinib plus R-CHOP improved EFS, PFS, and OS with manageable safety. In patients age 60 years or older, ibrutinib plus R-CHOP was associated with increased toxicity, leading to compromised R-CHOP administration and worse outcomes. Further investigation is warranted.

The HELIOS trial protocol: a phase III study of ibrutinib in combination with bendamustine and rituximab in relapsed/refractory chronic lymphocytic leukemia

Ibrutinib is an orally administered, covalent inhibitor of Bruton's tyrosine kinase with activity in B-cell malignancies based on Phase I/II studies. We describe the design and rationale for the Phase III HELIOS trial (trial registration: EudraCT No. 2012-000600-15; UTN No. U1111-1135-3745) investigating whether ibrutinib added to bendamustine and rituximab (BR) provides benefits over BR alone in patients with relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma. Eligible patients must have relapsed/refractory disease measurable on CT scan and meet ≥ 1 International Workshop on Chronic Lymphocytic Leukemia criterion for requiring treatment; patients with del(17p) are excluded. All patients receive BR (maximum six cycles) as background therapy and are randomized 1:1 to placebo or ibrutinib 420 mg/day. Treatment with ibrutinib or placebo will start concomitantly with BR and continue until disease progression or unacceptable toxicity. The primary end point is progression-free survival. Secondary end points include safety, objective response rate, overall survival, rate of minimal residual disease-negative remissions, and patient-reported outcomes. Tumor response will be assessed using the International Workshop on Chronic Lymphocytic Leukemia guidelines.

Development of the Bruton's tyrosine kinase inhibitor ibrutinib for B cell malignancies

Ibrutinib is a first-in-class oral covalent inhibitor of Bruton's tyrosine kinase that has demonstrated clinical benefit for many patients with B cell malignancies. Positive results in initial trials led the U.S. Food and Drug Administration to grant ibrutinib three breakthrough therapy designations for mantle cell lymphoma (MCL), del17p chronic lymphocytic leukemia (CLL), and Waldenström's macroglobulinemia (WM). Ibrutinib was approved for these three cancers within 14 months of the original U.S. approval. Additionally, ibrutinib is approved for patient subsets with MCL and/or CLL in >45 other countries. Via a unique mechanism of action, ibrutinib inhibits B cell signaling pathways that regulate the survival, proliferation, adhesion, and homing of cancerous cells. This marks a paradigm shift from the conventional cytotoxic chemotherapy approach to treating B cell malignancies. Ibrutinib continues to be evaluated across a range of B cell malignancies, either as single-agent therapy or in combination with other therapies, and continues to transform the lives of these patients.

A phase III study of ibrutinib in combination with bendamustine and rituximab (BR) in elderly patients with newly diagnosed mantle cell lymphoma (MCL)

TPS8613^

Background: MCL is a distinct subtype of B-cell non-Hodgkin lymphoma (NHL) accounting for approximately 6% of NHL diagnoses. Current immunochemotherapy followed by rituximab maintenance results in a prolonged duration of remission, but a constant relapse pattern is still observed. Ibrutinib, an oral Bruton's tyrosine kinase(BTK) inhibitor, demonstrated promising single-agent activity in 115 patients with relapsed or refractory MCL who were enrolled in the phase II study PCYC-1104. The overall response rate (ORR) was 68%, with 46% of patients achieving partial response (PR) and 22% achieving complete remission (CR) (Wang ASH 2012). Blum et al (ASH 2012) demonstrated that ibrutinib can be safely combined with BR in a phase I combination study in relapsed or refractory NHL and that it enhanced BR’s clinical activity with an ORR in 5 evaluable MCL patients of 100% (80% CR, 20% PR). These data suggest that combining ibrutinib with BR will improve the outcome of these patients. Methods: The SHINE study, PCI-32765MCL3002, is a phase III double-blind study of ibrutinib in combination with BR versus BR for the treatment of patients with newly diagnosed MCL. The study aims to enroll 520 patients (approximately 260 patients per arm). All patients will receive BR therapy for 6 cycles; those patients achieving a CR or PR will receive R maintenance for 2 years. In addition to BR and R, all patients will receive an oral daily dose of 560 mg ibrutinib or placebo concomitant with the chemotherapy and ongoing as a single agent until disease progression or unacceptable toxicity. The primary objective is to evaluate if the addition of ibrutinib to BR will result in prolongation of progression-free survival, with secondary objectives of ORR (CR+PR), CR rate, duration of response, safety, and overall survival. The study will enroll patients aged 65 years or older who are not suitable for high-dose chemotherapy. Key exclusion criteria include diagnosis or treatment for malignancy other than MCL, requirement for treatment with warfarin or equivalent vitamin K antagonists, and treatment with strong CYP3A4/5 inhibitors. Approximately 200 sites globally will enroll patients. Enrollment began in Q1 of 2013. Clinical trial information: NCT01776840.

An open-label phase II study of ibrutinib in patients with refractory follicular lymphoma

TPS8614^

Background: Follicular lymphoma (FL) is the second most common non-Hodgkin lymphoma (NHL) and comprises approximately 22% of all NHL cases. Most patients treated eventually relapse and subsequent responses and duration of responses become shorter. Patients ultimately become resistant to chemoimmunotherapy and repeated treatment-related toxicity commonly outweighs the benefit of treatment. Ibrutinib is a potent inhibitor of BTK (downstream of the B-cell receptor, BCR) that binds covalently to Cys-481 in the active site, abrogating intrinsic survival pathways (eg, ERK1/2, NF-kB, AKT) as well as survival signals from the microenvironment (eg, TNF family members: BAFF, CD40L; cytokines from T-cells: IL4, IL6, IL10, TNFα). Irish et al (2010) have also shown that up to 60% of FL patients display BCR signaling addiction. Early indications from study PCYC-04753 suggest activity of the BTK inhibitor ibrutinib in FL. Three CR and 3 PR were observed in 11 patients at a dose of 2.5 mg/kg or higher that achieved full BTK occupancy (Fowler, ASH 2012). Methods: The DAWN study, PCI-32765FLR2002, is a phase II, single-arm study of ibrutinib in refractory FL. The study aims to enroll 110 patients with chemoimmunotherapy-resistant FL. Patients will receive an oral daily dose of 560 mg ibrutinib. Patients must have been treated with at least 2 prior lines of therapy, at least 1 rituximab-containing combination chemotherapy regimen, and the last prior line of therapy included an anti-CD20 monoclonal antibody-containing chemotherapy regimen. The primary objective of the study is to evaluate the ORR (CR + PR), with secondary objectives of duration of response, progression-free survival, overall survival, and safety. To better understand the mechanism of action of ibrutinib, blood and tumor samples will be collected and, where feasible, characterized by GEP, SMA, IHC, or other technology as applicable. These evaluations aim to identify biomarkers associated with response or resistance to ibrutinib in subjects with FL and results may assist in the development of this drug in this and potentially other indications. Approximately 64 sites in the US and Europe will enroll patients. Enrollment began in 1Q of 2013. Clinical trial information: NCT01779791.

A method for assessing tumor response to therapy and more precisely guiding treatment decisions so as to improve survival

e13122

Background: The response of tumors to chemotherapy is monitored using imaging data or tumor markers and this quantitative data provides a rich source for an objective response assessment and treatment decisions. Responses are usually assessed as categorical variables based on percentage increase or decrease in tumor size. Methods: We have developed mathematical equations that describe efficacy as a continuous variable, enabling the extraction of the appropriate rate constants for tumor growth and regression (decay), designated g and d, respectively. Both are used to describe the rates of tumor growth and regression for the fraction of tumor that is growing despite treatment and the fraction dying as a result of therapy, respectively. Results: Using data from randomized phase III trials in kidney and breast cancer, multiple myeloma, and medullary thyroid carcinoma; as well as phase II trials in prostate cancer we have shown that: (1) values of g but not those of d are strongly correlated (negatively) with patient survival; (2) g can be discerned early in treatment, before growth is demonstrated clinically, providing an early efficacy measure; (3) g typically does not change over time, even over years, suggesting resistance is intrinsic and predictable and does not worsen over time; (4) effective therapies both increase d, and reduce g; and (5) in every cancer studied, the evidence suggests tumor growth reverts to its pre-treatment rate when chemotherapy is discontinued. Conclusions: The observation that g remains stable allows one to predict the most likely outcome of continued therapy. The evidence indicates that the increase in g occurring after treatment discontinuation is due to a resumption of a pre-treatment growth rate and not a change in biology. Our hypothesis is that if a favorable growth rate that slows tumor growth can be identified, survival might be improved if therapies that achieve this favorable growth rate are continued despite crossing conventional disease progression boundaries. We plan a prospective test of this model to provide a more informed decision and better survival outcome by maximizing the benefit obtained from approved therapies.

Abstract 765: Growth rates of tumors with numerous histologies remain constant while patients are on therapy

Tumor Growth Rates Remain Constant while Patients are on Therapy: Evidence that Continued Treatment does not Select for More Rapidly Growing Tumors Background: Increasingly, clinical investigators continue therapies with the hopes of improving survival. Where tumor growth is occurring, such a strategy will only succeed if the rate of tumor growth remains slow and unchanged while therapy continues. We tested this assumption using a large database of tumor measurements in patients on clinical trials. Results: We analyzed 2250 tumor growth curves from patients with advanced cancer and diagnoses of multiple myeloma, breast, renal, castration refractory prostate cancer (CRPC) or medullary thyroid carcinoma (MTC). Patients received various therapies: sunitinib, vandetanib, ixabepilone, capecitabine, bevacizumab, thalidomide, taxotere and interferon alpha. Measurements comprised serum markers (M-spike, PSA, CEA, calcitonin) and radiographic imaging. A simple computer-based model allowed extraction of statistically valid results for growth and decay rates (in over 85% of cases) and the fraction of surviving tumor in cases where marked responses occurred (up to 30% of MTC and CRPC). Analysis of the data obtained while patients were on therapy (including those still alive when the study closed and also those who died but survived long after therapy was discontinued) showed little evidence of an increase (0.3%) or a decrease (0.7%, most being MTC) in growth rate while they were treated, consistent with expansion of a pre-existing clone that is relatively resistant. Also in those MTC and CRPC cases where there was no detectable growth for a period of time, restoration of growth was consistent with expansion of a pre-existing, inherently-resistant clone, growing at a constant rate, rather than with emergence of a clone that has acquired resistance. In cases where therapy continued well beyond a year (median, 25%-75%: 498, 420-727 days) until just before death (interval duration median, 25% to 75%: 95, 63-129 days), there was neither evidence of acceleration nor the slowing that would be predicted by a Gompertz model. Many patients in whom therapy reduced the tumor growth rate to well below that of placebo, died sooner than expected. Modeling their survival data and their tumor growth rates suggested a return to the pre-therapy rate when treatment was discontinued. Conclusion: Data obtained on tumor growth rates for 2250 patients receiving therapy in clinical trials shows little evidence that tumor growth rates change while patients receive treatment. Analysis of the smaller sample of cases where death occurred within months after therapy ceased suggests that while growth rates remain constant while patients continue therapy, tumor growth returns to the pre-treatment rate when therapy is discontinued. These observations support a strategy of continuing therapy beyond the conventional criteria for discontinuation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 765. doi:1538-7445.AM2012-765

Inhibition of prostate cancer metastasis in vivo: a comparison of 1,23-dihydroxyvitamin D (calcitriol) and EB1089

The steroid hormone 1,25-dihydroxyvitamin D [1,25(OH)2D, also known as calcitriol] is known to inhibit the proliferation and to promote the differentiation of human prostate cancer cells. Additionally, we showed that 1,25(OH)2D markedly inhibits the invasiveness of human prostate cancer cells in vitro (G. G. Schwartz et al., Cancer Epidemiol. Biomark. Prev., 6: 727-732, 1997). These properties support the use of 1,25(OH)2D as differentiation therapy in prostate cancer. However, the use of 1,25(OH)2D in vivo is limited by the risk of hypercalcemia. We therefore compared the effects of 1,25(OH)2D and of EB1089, an analogue of 1,25(OH)2D with reduced calcemic effects, in an in vivo model of androgen-insensitive metastatic prostate cancer, the rat Dunning MAT LyLu prostate cancer model. Tumor growth and metastasis were studied using Copenhagen rats given s.c. injections of MAT LyLu cells. Fifty male rats were divided into five groups of 10 rats each. Four experimental groups received i.p. injections of low and high doses of 1,25(OH)2D and EB1089 (0.5 and 1.0 microg/kg, low and high, respectively). A control group received injections of vehicle only. Tumor volumes were measured three times per week. Rats were weighed weekly. The number of metastases to the lungs and the extent of hypercalcemia were evaluated. Compared with controls, tumor volumes were significantly smaller in all experimental groups. Similarly, the number of lung metastases (number of foci/lung) was reduced markedly by both 1,25(OH)2D and EB1089. Control rats developed 22.7 (+/- 1.98 SE) tumor foci per lung. Rats treated with 1,25(OH)2D and with EB1089 (1.0 microg/kg) developed 10.4 (+/- 2.81) and 7.70 (+/- 1.29) tumor foci, respectively (P < 0.001 and P < 0.0001, respectively; drug versus control). Compared with controls (10.79 +/- 0.1 mg/dl), serum calcium levels were significantly elevated in both 1,25(OH)2D and EB1089-treated rats (P < 0.01). However, EB1089 was significantly less calcemic than 1,25(OH)2D (12.59 +/- 0.21 mg/dl versus 14.47 +/- 0.46 mg/dl; 1.0 microg/kg; P < 0.001). Rats treated with 1,25(OH)2D showed marked weight loss: 20.0 +/- 1.9% and 26.3 +/- 1.7% of their initial weight (low and high doses, respectively, P < 0.001). Weight loss was significantly lower in rats treated with EB1089 at the high dose 8.4 (+/- 2.9) %. Moreover, rats treated with low-dose EB1089 gained 5.2 (+/- 3.7) % of their initial weight. In conclusion, 1,25(OH)2D and EB1089 showed marked and equivalent inhibition of prostate cancer metastasis in vivo. EB1089 was significantly less calcemic than 1,25(OH)2D and did not induce severe weight loss. This is the first report of a vitamin D analogue that significantly inhibits prostate cancer metastasis in vivo and that does so without producing cachexia or unacceptable hypercalcemia.

Antiproliferative effect of 1alpha,25-dihydroxyvitamin D3 in human prostate cancer cell line LNCaP involves reduction of cyclin-dependent kinase 2 activity and persistent G1 accumulation

1Alpha,25-dihydroxyvitamin D3 (1,25 D), the most active metabolite of vitamin D3, exerts antiproliferative and prodifferentiating effects on some human prostate cancer cell lines. We previously reported an inverse relationship between functional vitamin D receptor (VDR) levels and antiproliferative response to 1,25 D in two human prostate cancer cell lines, LNCaP and ALVA 31. Although LNCaP cells are far more sensitive to growth inhibition by 1,25 D than ALVA 31 cells, LNCaP express approximately half the number of VDR as ALVA 31. Two other human prostate cancer cell lines studied, PC3 and DU145, express lower levels of functional VDR and are relatively insensitive to growth inhibition by 1,25 D. In this report, we investigated potential mechanisms of the variable antiproliferative activity of 1,25 D. In PC3 cells stably expressing VDR [PC3(VDR)] at levels comparable to LNCaP, 1,25 D treatment resulted in only moderate growth inhibition. These results further support the contention that VDR expression, although required, is not sufficient for maximal growth suppression by 1,25 D, as is exhibited by LNCaP cells. We did not detect 1,25 D-mediated DNA fragmentation after 4 days of 1,25 D treatment in either LNCaP or ALVA 31 cells. This result suggests that variability in 1,25 D sensitivity does not derive from differences in the capacity of these cells to undergo apoptosis in response to 1,25 D. Flow cytometry of propidium iodine-stained cells revealed that 48 h 1,25 D treatment of LNCaP cells resulted in a 2-fold decrease of cells in G2/M plus S phases and accumulation of LNCaP cells in the G1/G0 phase. This effect persisted for 72 h after 1,25 D removal. In contrast, 1,25 D did not significantly alter the cell cycle distribution of ALVA 31 or PC3(VDR) cells. Consistent with accumulation of cells in G1/G0, 1,25 D treatment of LNCaP cells resulted in decreased retinoblastoma protein phosphorylation, repressed E2F transcriptional activity, increased levels of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1, CIP1), and decreased CDK2 activity. However, p21 messenger RNA levels were not altered, suggesting translational or posttranslational regulation of p21 by 1,25 D. In contrast, p21 was not detected in ALVA 31 or PC3(VDR) and was not induced by 1,25 D, consistent with the failure of 1,25 D to influence cell cycle distribution in these cells. These results suggest that variability in sensitivity to the antiproliferative effects of 1,25 D among prostate cancer cells is dependent, at least in part, on the integrity of the retinoblastoma pathway and in particular on p21 expression and 1,25 D regulation of CDK2 activity.

Vitamin D receptor content and transcriptional activity do not fully predict antiproliferative effects of vitamin D in human prostate cancer cell lines

Prostate cancer cell lines exhibit variable growth suppression by the hormonal form of vitamin D3, 1,25-Dihydroxyvitamin D3 [1,25 (OH)2D] (1,25 D3). To understand the molecular basis for this differential sensitivity to 1,25 D3, we compared growth response to 1,25 d3, vitamin D receptor (VDR) content and VDR transcriptional activity in four well-characterized human prostate cancer cell lines: LNCaP, DU145, PC-3 and ALVA-31. In PC-3 and DU145 cells, relative lack of growth inhibition by 1,25 D3 (< 10% inhibition) correlates with very low levels of VDR (9-15 fmol/mg protein) compared to classical vitamin D3 target tissues (approximately 75-200 fmol/mg protein). Transfection of DU145 and PC-3 cells with a VDR cDNA expression vector is sufficient to establish growth sensitivity to 1,25 D3, suggesting that low VDR levels are responsible for the failure of these cell lines to respond to 1,24 D3. LNCaP cells are highly sensitive to growth inhibition by 1.25 D3 (approximately 55% inhibition) and contain approximately 2-3-fold more VDR (25 fmol/mg) than the relatively 1,25 D3-insensitive PC-3 and DU145 cell lines. However, ALVA-31 cells display less than 20% growth inhibition to 1.25 D3 although they contain the highest levels of VDR (45 fmol/mg) of the four cell lines. Thus, sensitivity to growth inhibition by 1,25 D3 does not correlate with VDR content in ALVA-31 and LNCaP cells. This lack of correlation between VDR density and growth responses to 1,25 D3 led us to investigate VDR-mediated gene transcription in these cell lines. We employed two different naturally occurring vitamin D response elements (VDREs) linked to a reporter gene. Reporter gene activation by 1,25 D3 correlated well with VDR content in all four cell lines. Therefore, compared to LNCaP cells, decreased sensitivity of ALVA-31 to growth inhibition by 1,25 D3 is not due to a decrease in the general transcriptional activity of VDR. We conclude that growth inhibition by 1,25 D3 in prostate cancer cells requires VDR but that this response is modulated by non-receptor factors that are cell line-specific.