Open-label, phase I, pharmacokinetic studies of abiraterone acetate in healthy men

From Dose-Finding to Dose-Optimization in Early-Phase oncology clinical trials

Dose optimization in Phase I oncology trials balances therapeutic efficacy and patient safety. Traditional dose-escalation methods, such as the 3 + 3 design, primarily focus on safety, often resulting in prolonged exposure to subtherapeutic or excessively toxic doses. Additionally, these methods may fail to account for modern therapies' complex pharmacokinetics and pharmacodynamics, including targeted agents and immunotherapies. Contemporary approaches address these gaps by incorporating biomarkers, pharmacokinetic profiling, and patient-reported outcomes to guide personalized dosing strategies. Such methods improve the precision of dose selection and promote individualized cancer care. This review underscores the importance of distinguishing between dose-finding and dose optimization, advocating for designs that integrate patient perspectives and pharmacologic insights from early-phase trials. Additionally, we highlight the challenges of traditional methodologies and the importance of simplifying complex designs without compromising their scientific rigor. By embracing innovative approaches and patient-centered metrics, Phase I trials can evolve beyond safety assessments to expedite the delivery of effective and tailored cancer therapies.

Formulating abiraterone acetate-HPMCAS-based amorphous solid dispersions: insights into in vitro and biorelevant dissolution assessments and pharmacokinetic evaluations

Abiraterone acetate (ABTA) is used as a primary treatment for metastatic castration-resistant prostate cancer. Its low aqueous solubility results in inadequate dissolution and poor oral bioavailability (<10%), necessitating the consumption of large doses of ABTA (1000 mg per day) for desired efficacy. The aim of this study is to enhance the solubility, dissolution, and bioavailability of ABTA through amorphous solid dispersions (SDs). ABTA-SD was prepared via a solvent granulation method with different grades of hydroxypropyl methylcellulose acetate succinate (HPMCAS 716 and 912). The theoretical solubility parameter between ABTA and HPMCAS was below 7 MPa1/2, indicating miscibility between the drug and the polymer according to the Hansen solubility parameter. HPMCAS showed a remarkable recrystallization inhibition of up to 180 min compared to the free drug (10 min), maintaining the soluble drug in supersaturation state and exhibiting the "spring and parachute" phenomenon. ABTA-SD exhibited a higher solubility (1.16-fold to 52-fold) in different media than free ABTA. The results of DSC, PXRD, ATR-FTIR, and FE-SEM indicated that the crystallinity of ABTA was completely transformed to an amorphous form and maintained in the SD formulation. In vitro and bio-relevant dissolution behavior of ABTA was studied in various dissolution media, indicating the higher dissolution of ABTA-SD than that of free ABTA. The pharmacokinetic study conducted in Wistar rats revealed that C max and AUC0-t of the optimized ABTA-SD formulation were significantly enhanced by 1.92-fold and 2.87-fold, respectively, compared to those of free ABTA.

Androgen receptor pathway inhibitors and drug-drug interactions in prostate cancer

Prostate cancer represents a major global health challenge, necessitating efficacious therapeutic strategies. Androgen receptor pathway inhibitors (ARPIs) have become central to prostate cancer treatment, demonstrating significant effectiveness in both metastatic and non-metastatic contexts. Abiraterone acetate, by inhibiting androgen synthesis, deprives cancer cells androgens necessary for growth, while second-generation androgen receptor (AR) antagonists disrupt AR signaling by blocking AR binding, thereby impeding tumor progression. Given the predominance of prostate cancer in the elderly, who often present with multiple comorbidities requiring complex pharmacological regimens, the potential for drug-drug interactions with ARPIs is a critical concern. These interactions, particularly through pathways like CYP2D6 inhibition by abiraterone and CYP3A4 induction by enzalutamide and apalutamide, necessitate a thorough understanding to optimize therapeutic outcomes and minimize adverse effects. This review aims to delineate the efficacy of ARPIs in prostate cancer management and elucidate their interaction with common medications, highlighting the importance of vigilant drug management to optimize patient care.

Making the Case for Autophagy Inhibition as a Therapeutic Strategy in Combination with Androgen-Targeted Therapies in Prostate Cancer

Androgen receptor targeting remains the primary therapeutic strategy in prostate cancer, encompassing androgen biosynthesis inhibitors and androgen receptor antagonists. While both androgen-receptor-positive and "castration-resistant" prostate cancer are responsive to these approaches, the development of resistance is an almost inevitable outcome leading to the castration-resistant form of the disease. Given that "cytoprotective" autophagy is considered to be a predominant mechanism of resistance to various chemotherapeutic agents as well as to radiation in the cancer literature, the purpose of this review is to evaluate whether autophagy plays a central role in limiting the utility of androgen deprivation therapies in prostate cancer. Unlike most of our previous reports, where multiple functional forms of autophagy were identified, making it difficult if not impossible to propose autophagy inhibition as a therapeutic strategy, the cytoprotective form of autophagy appears to predominate in the case of androgen deprivation therapies. This opens a potential pathway for improving the outcomes for prostate cancer patients once effective and reliable pharmacological autophagy inhibitors have been developed.

Unraveling Complexities in the Absorption and Disposition Kinetics of Abiraterone via Iterative PBPK Model Development and Refinement

BACKGROUND AND OBJECTIVE

Abiraterone is a first-in-class inhibitor of cytochrome P450 17A1 (CYP17A1), and its pharmacokinetic (PK) profile is susceptible to intrinsic and extrinsic variabilities. Potential associations between abiraterone concentrations and pharmacodynamic consequences in prostate cancer may demand further dosage optimization to balance therapeutic outcomes. Consequently, we aim to develop a physiologically based pharmacokinetic (PBPK) model for abiraterone via a middle-out approach to prospectively interrogate the untested, albeit clinically relevant, scenarios.

METHODS

To characterize in vivo hydrolysis of prodrug abiraterone acetate (AA) and supersaturation of abiraterone, in vitro aqueous solubility data, biorelevant measurements, and supersaturation and precipitation parameters were utilized for mechanistic absorption simulation. CYP3A4-mediated N-oxidation and sulfotransferase 2A1-catalyzed sulfation of abiraterone were subsequently quantified in human liver subcellular systems. Iterative PBPK model refinement involved evaluation of potential organic anion transporting polypeptide (OATP)-mediated abiraterone uptake in transfected cells in the absence and presence of albumin.

RESULTS

The developed PBPK model recapitulated the duodenal concentration-time profile of both AA and abiraterone after simulated AA administration. Our findings established abiraterone as a substrate of hepatic OATP1B3 to recapitulate its unbound metabolic intrinsic clearance. Further consideration of a transporter-induced protein-binding shift established accurate translational scaling factors and extrapolated the sinusoidal uptake process. Subsequent simulations effectively predicted the PK of abiraterone upon single and multiple dosing.

CONCLUSION

Our systematic development of the abiraterone PBPK model has demonstrated its application for the prospective interrogation of the individual or combined influences of potential interindividual variabilities influencing the systemic exposure of abiraterone.

Pharmaceutical Product Characterization and Manufacturability of Surfactant-Enriched Oil Marbles with Abiraterone Acetate

The present study investigates the physicochemical properties and stability of a novel lipid-based formulation-surfactant-enriched oil marbles containing abiraterone acetate. While the biopharmaceutical performance of this formulation has been reported recently, this study aims to fill the gap between a promising in vivo performance and industrial applicability. A series of techniques were employed to assess the solid-state characteristics of oil marble cores along with their physicochemical properties upon stability testing. The chemical stability of abiraterone acetate in the formulation was also investigated. The core of the formulation was found to be stable both physically and chemically over 12 months of storage. The in vitro performance of stressed samples was evaluated using a dissolution experiment. The formulation has successfully self-emulsified upon incubation in bio-relevant media, resulting in a fast and complete API release. An important issue connected with the excipient used as a covering material of oil marbles has been identified. The seemingly insignificant water sorption caused agglomeration of the oil marbles and consequently compromised the dissolution rate in some of the stressed samples. Replacing HPMC with lactose as a covering material resulted in more favorable properties upon storage. Overall, it has been shown that oil marbles are an industrially applicable concept of the solidified lipid-based formulation.

Is Low-Dose Abiraterone for Prostate Cancer An Attractive Strategy for Limited Resource Settings?

Abiraterone acetate in combination with prednisone is approved for locally advanced as well as metastatic (hormone-sensitive and castrate-resistant) prostate cancer, with overall or disease-free survival gains in suitable patients. Long-term use poses a significant financial strain on the self-paying patients as well as the national health insurance schemes. Abiraterone is known to be a drug with a high “food effect” with increased bioavailability following high fat diet. Some retrospective series and phase 1 and 2 clinical studies have explored the use of low-dose abiraterone (at 25% of standard dose) with high fat meal with similar bioavailability and biochemical response to the standard drug dose. We review and report the available literature for this approach and discuss the financial and scientific implications of the same.

Development, Characterization, and Optimization of a Novel Abiraterone Acetate Formulation to Improve Biopharmaceutical Attributes Aided by Pharmacokinetic Modelling

Abiraterone acetate has very low bioavailability and drastic food effect to warrant a dosing regimen under fasting state only. Therefore, we aimed to develop and optimize a liquisolid compact formulation of abiraterone acetate to improve biopharmaceutical attributes aided by pharmacokinetic modelling and achieve dose reduction with no food effect on the formulation. Preliminary studies highlighted the importance of the selection of olive oil as a compatible vehicle. The pharmacokinetic model, integrated with gastrointestinal physiology, was used to predict fasted and fed state pharmacokinetic parameters. Optimization of the liquisolid formulation containing abiraterone acetate was carried at more than five times lower dose, i.e. 190 mg, compared to 1000 mg. A central composite design (CCD) was used to identify optimal levels of formulation factors, namely the amount of vehicle (olive oil), the amount of coating agent (silicon dioxide), and the amount of surfactant (polysorbate 80). Graphical optimization using the selected models in conjunction with maximization of the desirability was used to identify the optimized liquisolid formulation. The predicted pharmacokinetic parameters (fasted C_max 901.83 ng/mL, fasted AUC_inf 2723.82 ng·h/mL, fed C_max 1024.34 ng/mL, and fed AUC_inf 3041.83 ng·h/mL) of the optimized formulation were acceptable. Overall, the liquisolid compact formulation of abiraterone acetate was successfully developed and optimized. In vitro solubility and dissolution results aided by pharmacokinetic modelling also showed improved predicted bioavailability with greater than five times reduction in dose and elimination of food effect.

Slow-, Tight-Binding Inhibition of CYP17A1 by Abiraterone Redefines Its Kinetic Selectivity and Dosing Regimen

Substantial evidence underscores the clinical efficacy of inhibiting CYP17A1-mediated androgen biosynthesis by abiraterone for treatment of prostate oncology. Previous structural analysis and in vitro assays revealed inconsistencies surrounding the nature and potency of CYP17A1 inhibition by abiraterone. Here, we establish that abiraterone is a slow-, tight-binding inhibitor of CYP17A1, with initial weak binding preceding the subsequent slow isomerization to a high-affinity CYP17A1-abiraterone complex. The in vitro inhibition constant of the final high-affinity CYP17A1-abiraterone complex ( ( K i * = 0.39 nM )yielded a binding free energy of -12.8 kcal/mol that was quantitatively consistent with the in silico prediction of -14.5 kcal/mol. Prolonged suppression of dehydroepiandrosterone (DHEA) concentrations observed in VCaP cells after abiraterone washout corroborated its protracted CYP17A1 engagement. Molecular dynamics simulations illuminated potential structural determinants underlying the rapid reversible binding characterizing the two-step induced-fit model. Given the extended residence time (42 hours) of abiraterone within the CYP17A1 active site, in silico simulations demonstrated sustained target engagement even when most abiraterone has been eliminated systemically. Subsequent pharmacokinetic-pharmacodynamic (PK-PD) modeling linking time-dependent CYP17A1 occupancy to in vitro steroidogenic dynamics predicted comparable suppression of downstream DHEA-sulfate at both 1000- and 500-mg doses of abiraterone acetate. This enabled mechanistic rationalization of a clinically reported PK-PD disconnect, in which equipotent reduction of downstream plasma DHEA-sulfate levels was achieved despite a lower systemic exposure of abiraterone. Our novel findings provide the impetus for re-evaluating the current dosing paradigm of abiraterone with the aim of preserving PD efficacy while mitigating its dose-dependent adverse effects and financial burden. SIGNIFICANCE STATEMENT: With the advent of novel molecularly targeted anticancer modalities, it is becoming increasingly evident that optimal dose selection must necessarily be predicated on mechanistic characterization of the relationships between target exposure, drug-target interactions, and pharmacodynamic endpoints. Nevertheless, efficacy has always been perceived as being exclusively synonymous with affinity-based measurements of drug-target binding. This work demonstrates how elucidating the slow-, tight-binding inhibition of CYP17A1 by abiraterone via in vitro and in silico analyses was pivotal in establishing the role of kinetic selectivity in mediating time-dependent CYP17A1 engagement and eventually downstream efficacy outcomes.

Enhancement of abiraterone acetate oral bioavailability by supersaturated-silica lipid hybrids

Abiraterone acetate (AbA) has an oral bioavailability of <10% due to its poor water solubility. Here we investigate the performance of silica-lipid hybrids (SLH) and supersaturated SLH (super-SLH) in improving oral bioavailability of AbA. Specifically, we investigate the influence of lipid type and AbA saturation level of the equilibrium solubility in the lipid (S_eq), and explore in vitro-in vivo correlation (IVIVC). An oral pharmacokinetic study was conducted in fasted Sprague-Dawley rats. Suspensions of the formulations were administered via oral gavage at an AbA dose of 25 mg/kg. Plasma samples were collected and analyzed for drug content. SLH with a saturation level of 90% S_eq enhanced the oral bioavailability of unformulated AbA by 31-fold, and super-SLH with saturation levels of 150, 200 and 250% S_eq, enhanced the bioavailability by 11, 10 and 7-fold, respectively. In comparison with the commercial product Zytiga, SLH (90% S_eq) increased the oral bioavailability 1.43-fold whereas super-SLH showed no improvement. A reasonable IVIVC existed between the performance of unformulated AbA, SLH and super-SLH, in the in vitro lipolysis and in vivo oral pharmacokinetic studies. SLH and super-SLH significantly enhanced the oral bioavailability of AbA. Additionally, supersaturation of SLH improved drug loading but did not correlate with enhanced AbA bioavailability.

Preclinical evaluation of new formulation concepts for abiraterone acetate bioavailability enhancement based on the inhibition of pH-induced precipitation

Abiraterone acetate is a potent drug used for the treatment of metastatic castration resistant prostate cancer. However, currently marketed product containing crystalline abiraterone acetate exhibits strong positive food effect which results in strict dosing regimen. In the present work, a rational approach towards design of novel abiraterone acetate formulations that would allow increased bioavailability on a fasting stomach and thus decreased food effect is presented. Precipitation experiments in biorelevant media were designed to assess pH induced precipitation of the drug and a pool of polymeric excipients was then screened for their potential to inhibit precipitation. The best performing polymeric excipients were subsequently used as carriers for the preparation of amorphous solid dispersions. Two main approaches were followed in order to formulate the drug. The first approach relies on the suppression of precipitation from a supersaturated solution whereas the second one is based on the hypothesis that when the release of the drug is tuned, optimal uptake of the drug can be reached. Optimized formulation prototypes were tested in a rat animal model in an incomplete block, randomized bioequivalence study to assess their relative bioavailability under fasting conditions. We show that both formulation approaches lead to increased bioavailability of abiraterone acetate on a fasting stomach with bioavailability in rats being enhanced up to 250% compared to the original drug product containing crystalline drug.

Liver tests increase on abiraterone acetate in men with metastatic prostate cancer: Natural history, management and outcome

BACKGROUND

Abiraterone acetate (abiraterone) combined with prednisone is a standard of care in metastatic castration-resistant prostate cancer. Recently, benefit in overall survival was reported in metastatic castration-sensitive prostate cancer also, and an extension of indication has been granted. Abiraterone is seldom associated with liver toxicity. The clinical management and the outcome of patients with transaminase increase while on abiraterone have not been described.

PATIENTS AND METHOD

We identified 25 men with metastatic prostate cancer and liver function test disorders occurring while on abiraterone treatment from December 2009 to September 2017 in three oncology centres in France.

RESULTS

Forty-six liver disorder events occurred in 25 patients while on abiraterone treatment. The median age at liver function test increase was 67 (55-85) years. The incidence of aspartate aminotransférase (AST) (24 events) and that of alanine aminotransférase (ALT) (22 events) increases were similar. Liver toxicity was of grade 1, 2 and 3 (Common Terminology Criteria for Adverse Events. version 4) in 7 (32%), 6 (27%) and 9 (41%) patients for ALT, and in 12 (50%), 6 (25%) and 6 (25%) for AST, respectively. The median time from abiraterone initiation to the detection of liver toxicity was 7.1 (4-95) weeks. The median time from highest ALT/AST increase to normalisation was 6.2 [2-14] weeks. In 13 patients (52%), liver tests spontaneously returned to baseline values, while abiraterone was continued at full dose.

CONCLUSION

Liver function test increase is a rare event that typically occurs within the first two months on abiraterone. Most patients experience normalisation of the tests, either spontaneously or after dose reduction/discontinuation.

Inter- and intra-patient variability in pharmacokinetics of abiraterone acetate in metastatic prostate cancer

PURPOSE

This study examined the inter- and intra-patient variability in pharmacokinetics of AA and its metabolites abiraterone and Δ(4)-abiraterone (D4A), and potential contributing factors.

METHODS

AA administered daily for ≥4 weeks concurrently with androgen deprivation therapy (ADT) for mCRPC were included. Pharmacokinetic evaluation was performed at two consecutive visits at least 4 weeks apart. Plasma samples were collected 24 h after last dose of AA to obtain drug trough level (DTL) of two active metabolites, abiraterone and D4A.

RESULTS

39 plasma samples were obtained from 22 patients, with 17 patients had repeat DTL measurement. Considerable inter-patient variability in DTL was seen, with initial DTL for abiraterone ranging between 1.5 and 25.4 ng/ml (CV 61%) and for D4A between 0.2 and 2.5 ng/ml (CV 61%). Intra-patient variability in DTL for abiraterone varied between 0.85 and 336% and for D4A between 1.14 and 199%. There was no increase in AA exposure with use of dexamethasone (n = 5; DTL 13.9) compared with prednisone (n = 17; DTL 11.0 p = 0.5), dosing in fasted state (n = 13, DTL 12.1) compared to dosing in fed state (n = 9; DTL 11.1, p = 0.8), or chemotherapy-exposed (n = 10; DTL 8.9) compared to chemotherapy naïve (n = 12; DTL 14.0, p = 0.1).

CONCLUSIONS

Our cohort demonstrated high inter- and intra-patient variability in both abiraterone and D4A with fixed dosing of AA, with no effect from choice of corticosteroids, prior use of chemotherapy, or dosing in fasting state. Monitoring DTL of AA may be necessary to minimise risk of patients being under-dosed and earlier development of resistance.

Identification of Galeterone and Abiraterone as Inhibitors of Dehydroepiandrosterone Sulfonation Catalyzed by Human Hepatic Cytosol, SULT2A1, SULT2B1b, and SULT1E1

Galeterone and abiraterone acetate are antiandrogens developed for the treatment of metastatic castration-resistant prostate cancer. In the present study, we investigated the effect of these drugs on dehydroepiandrosterone (DHEA) sulfonation catalyzed by human liver and intestinal cytosols and human recombinant sulfotransferase enzymes (SULT2A1, SULT2B1b, and SULT2E1) and compared their effects to those of other antiandrogens (cyproterone acetate, spironolactone, and danazol). Each of these chemicals (10 μM) inhibited DHEA sulfonation catalyzed by human liver and intestinal cytosols. Enzyme kinetic analysis showed that galeterone and abiraterone acetate inhibited human liver cytosolic DHEA sulfonation with apparent Ki values at submicromolar concentrations, whereas cyproterone acetate, spironolactone, and danazol inhibited it with apparent Ki values at low micromolar concentrations. The temporal pattern of abiraterone formation and abiraterone acetate depletion suggested that the metabolite abiraterone, not the parent drug abiraterone acetate, was responsible for the inhibition of DHEA sulfonation in incubations containing human liver cytosol and abiraterone acetate. Consistent with this proposal, similar apparent Ki values were obtained, regardless of whether abiraterone or abiraterone acetate was added to the enzymatic incubation. Abiraterone was more effective than abiraterone acetate in inhibiting DHEA sulfonation when catalyzed by human recombinant SULT2A1 or SULT2B1b. In conclusion, galeterone and abiraterone are novel inhibitors of DHEA sulfonation, as determined in enzymatic incubations containing human tissue cytosol (liver or intestinal) or human recombinant SULT enzyme (SULT2A1, SULT2B1b, or SULT1E1). Our findings on galeterone and abiraterone may have implications in drug-drug interactions and biosynthesis of steroid hormones.

Association of Tissue Abiraterone Levels and SLCO Genotype with Intraprostatic Steroids and Pathologic Response in Men with High-Risk Localized Prostate Cancer

Purpose: Germline variation in solute carrier organic anion (SLCO) genes influences cellular steroid uptake and is associated with prostate cancer outcomes. We hypothesized that, due to its steroidal structure, the CYP17A inhibitor abiraterone may undergo transport by SLCO-encoded transporters and that SLCO gene variation may influence intracellular abiraterone levels and outcomes.Experimental Design: Steroid and abiraterone levels were measured in serum and tissue from 58 men with localized prostate cancer in a clinical trial of LHRH agonist plus abiraterone acetate plus prednisone for 24 weeks prior to prostatectomy. Germline DNA was genotyped for 13 SNPs in six SLCO genes.Results: Abiraterone levels spanned a broad range (serum median 28 ng/mL, 108 nmol/L; tissue median 77 ng/mL, 271 nmol/L) and were correlated (r = 0.355, P = 0.001). Levels correlated positively with steroids upstream of CYP17A (pregnenolone, progesterone), and inversely with steroids downstream of CYP17A (DHEA, AED, testosterone). Serum PSA and tumor volumes were higher in men with undetectable versus detectable tissue abiraterone at prostatectomy (median 0.10 vs. 0.03 ng/dL, P = 0.02; 1.28 vs. 0.44 cc, P = 0.09, respectively). SNPs in SLCO2B1 associated with significant differences in tissue abiraterone (rs1789693, P = 0.0008; rs12422149, P = 0.03) and higher rates of minimal residual disease (tumor volume < 0.5 cc; rs1789693, 67% vs. 27%, P = 0.009; rs1077858, 46% vs. 0%, P = 0.03). LNCaP cells expressing SLCO2B1 showed two- to fourfold higher abiraterone levels compared with vector controls (P < 0.05).Conclusions: Intraprostatic abiraterone levels and genetic variation in SLCO genes are associated with pathologic responses in high-risk localized prostate cancer. Variation in SLCO genes may serve as predictors of response to abiraterone treatment. Clin Cancer Res; 23(16); 4592-601. ©2017 AACR.

Relation between plasma trough concentration of abiraterone and prostate-specific antigen response in metastatic castration-resistant prostate cancer patients

BACKGROUND

Abiraterone (ABI) is a major oral agent for the treatment of metastatic castration-resistant prostate cancer (mCRPC) patients but its systemic exposure is subject to a large inter-individual variability. We aimed to explore the relationship between ABI trough plasma concentration and prostate-specific antigen (PSA) response in mCRPC patients and to identify the critical determinants for its activity.

PATIENTS AND METHODS

This is a monocentric prospective observational study in mCRPC patients treated with ABI. The plasmatic concentration of ABI at steady state was measured using liquid chromatography with fluorescence detection. The primary objective was to study the relationship between mean ABI plasma exposure (ABI C_min) and 3-month PSA response.

RESULTS

From 2012 to 2016, 61 mCRPC patients were eligible for pharmacokinetic/pharmacodynamic assessment. Thirty-eight patients experienced PSA response (62%, [confidence interval {CI} 95% 50-78]). In univariate analysis, ABI C_min was 1.5-fold higher in responders: 12.0 ng/mL (CI 95% 9.4-15.6) versus 8.0 ng/mL (CI 95% 5.8-11.6; P = 0.0015). In multivariate analysis, only ABI C_min was independently associated with PSA response (odds ratio = 1.12 [CI 95% 1.01-1.25], P = 0.004). By receiver operating characteristic analysis, the optimal threshold for ABI C_min was 8.4 ng/mL. Progression-free survival (PFS) was significantly higher in patients with ABI C_min above 8.4 ng/mL (hazard ratio 0.55, [CI 95% 0.31-0.99], 12.2 [CI 95% 9.2-19.5] versus 7.4 [CI 95% 5.5-14.7] months otherwise, P = 0.044).

CONCLUSIONS

We showed that ABI trough concentration correlates with PSA response and PFS. Moreover, we could determine a cut-off value of plasmatic concentration for PSA response. Altogether, ABI concentration monitoring appears as a new approach to improve clinical outcome in mCPRC patients.

In Vitro and In Vivo Drug-Drug Interaction Studies to Assess the Effect of Abiraterone Acetate, Abiraterone, and Metabolites of Abiraterone on CYP2C8 Activity

Abiraterone acetate, the prodrug of the cytochrome P450 C17 inhibitor abiraterone, plus prednisone is approved for treatment of metastatic castration-resistant prostate cancer. We explored whether abiraterone interacts with drugs metabolized by CYP2C8, an enzyme responsible for the metabolism of many drugs. Abiraterone acetate and abiraterone and its major metabolites, abiraterone sulfate and abiraterone sulfate N-oxide, inhibited CYP2C8 in human liver microsomes, with IC50 values near or below the peak total concentrations observed in patients with metastatic castration-resistant prostate cancer (IC50 values: 1.3-3.0 µM, 1.6-2.9 µM, 0.044-0.15 µM, and 5.4-5.9 µM, respectively). CYP2C8 inhibition was reversible and time-independent. To explore the clinical relevance of the in vitro data, an open-label, single-center study was conducted comprising 16 healthy male subjects who received a single 15-mg dose of the CYP2C8 substrate pioglitazone on day 1 and again 1 hour after the administration of abiraterone acetate 1000 mg on day 8. Plasma concentrations of pioglitazone, its active M-III (keto derivative) and M-IV (hydroxyl derivative) metabolites, and abiraterone were determined for up to 72 hours after each dose. Abiraterone acetate increased exposure to pioglitazone; the geometric mean ratio (day 8/day 1) was 125 [90% confidence interval (CI), 99.9-156] for Cmax and 146 (90% CI, 126-171) for AUClast Exposure to M-III and M-IV was reduced by 10% to 13%. Plasma abiraterone concentrations were consistent with previous studies. These results show that abiraterone only weakly inhibits CYP2C8 in vivo.

Enzymatic and Nonenzymatic Electrochemical Interaction of Abiraterone (Antiprostate Cancer Drug) with Multiwalled Carbon Nanotube Bioelectrodes

Unexplored electrochemical behavior of abiraterone, a recent and widely used prostate cancer drug, in interaction with cytochrome P450 3A4 (CYP3A4) enzyme and multiwalled carbon nanotubes (MWCNTs) is investigated in this work. The results reported in this work are significant for personalized medicine and point-of-care chemical treatment, especially to improve the life expectancy and quality of life of patients with prostate-cancer. To this purpose, enzymatic and nonenzymatic electrochemical biosensors were developed and characterized with different concentrations of abiraterone. Nonenzymatic biosensors were functionalized with MWCNTs as a catalyst for signal enhancement, while enzymatic biosensors have been obtained with CYP3A4 protein immobilized on MWCNTs as recognition biomolecule. Enzymatic electrochemical experiments demonstrated an inhibition effect on the CYP3A4, clearly observed as a diminished electrocatalytic activity of the enzyme. Electrochemical responses of nonenzymatic biosensors clearly demonstrated the direct electroactivity of abiraterone when reacting with MWCNT as well as an electrode-fouling effect.

Pharmacokinetics of abiraterone in healthy Japanese men: dose-proportionality and effect of food timing

PURPOSE

Abiraterone acetate (AA) was recently approved for castration-resistant prostate cancer in Japan. Two phase 1 studies were conducted to assess the pharmacokinetics of abiraterone after single-dose administration in Japanese healthy men and to evaluate the effects of food timing on abiraterone pharmacokinetics after single-dose administration of AA in Japanese and Caucasian healthy men.

METHODS

In the dose-proportionality study, subjects (n = 30 Japanese) were randomly assigned to receive single doses of 250, 500, and 1,000 mg AA, and in the food-timing study, subjects (n = 22 Japanese and n = 23 Caucasian) randomly received single doses of 1,000 mg AA under fasted (overnight) and three different modified fasting conditions.

RESULTS

Mean C(max) and AUC(∞) for abiraterone increased dose-dependently in Japanese healthy men; however, 90 % confidential interval (CI) was outside the predefined dose-proportionality criteria. Based on geometric mean ratios and 90 % CIs (versus overnight fasting condition), abiraterone exposure (AUC) increased significantly with dosing 1 h premeal, 2 h postmeal, or in between two meals 4 h apart by 57 %, 595 %, and 649 %, respectively.

CONCLUSION

No clinically meaningful difference was observed in the pharmacokinetics of abiraterone between Caucasian and Japanese subjects.

Phase-1 study of abiraterone acetate in chemotherapy-naïve Japanese patients with castration-resistant prostate cancer

Persistent androgen synthesis under castration status in adrenal gland, testes and tumor cells is thought to be one of the major causes of development and progression of castration-resistant prostate cancer (CRPC). Abiraterone acetate (AA), the prodrug of abiraterone, which is an inhibitor of androgen synthesis enzymes, was evaluated for pharmacokinetics, pharmacodynamics, preliminary efficacy and safety in Japanese patients with CRPC in a phase-1, open-label and dose-escalation study. Chemotherapy-naïve Japanese CRPC patients (N = 27) received one of four AA daily doses (250 mg [n = 9], 500 mg [n = 6], 1000 [1 h premeal] mg [n = 6] and 1000 [2 h postmeal] mg [n = 6]) continuously through 28-day treatment cycles. In the first cycle, AA monotherapy was given on days 1–7 for pharmacokinetics, and AA plus prednisone (5 mg twice daily) from days 8 to 28. Of 27 patients, 9 continued treatment with AA until the data cut-off date (18 July 2013). Over the evaluated dose range, plasma abiraterone concentrations increased with dose, with median t_max 2–3 h. At each dose level, mean serum corticosterone concentrations increased, while testosterone and dehydroepiandrosterone sulfate concentrations rapidly decreased following a single AA dose and were further reduced to near the quantification limit on day 8 regardless of the dose. At least 3 patients from each dose-group experienced ≥50% prostate-specific antigen reduction, suggesting clinical benefit from AA in Japanese CRPC patients. AA was generally well-tolerated, and, therefore, the recommended AA dosage regimen in Japanese CRPC patients is 1000 mg oral dose under modified fasting conditions (at least 1 h premeal or 2 h postmeal). This study is registered at ClinicalTrials.gov: NCT01186484.

A phase I, open-label, single-dose, mass balance study of 14C-labeled abiraterone acetate in healthy male subjects

1. Metabolic disposition of (14)C-abiraterone acetate (AA), a prodrug of abiraterone was assessed in a phase I, open-label, single-dose (1000 mg, approximately 100 μCi) study in healthy males (18-55 years, N = 8). Blood, urine, and faecal samples were obtained at specified timepoints for determination of abiraterone concentrations in the plasma, total radioactivity (TR), and the metabolite profile. 2. Most plasma AA concentrations were below the limit of quantification. The mean maximum plasma concentration (Cmax) of abiraterone was 10.4 ng/mL, mean area under the plasma concentration-time curve (AUC) from 0 to the last measurable plasma concentration (AUC0-last) was 74.8 ng·h/mL. The exposures for TR in plasma (Cmax = 3429 ng·eq/mL; AUC0-last = 26,683 ng eq·h/mL) and whole blood (Cmax = 1836 ng·eq/mL; AUC0-last = 12,162 ng·eq·h/mL) were >300-fold higher than abiraterone exposure in plasma. The majority of TR resided in the plasma compartment of blood. 3. Main circulating metabolites were abiraterone sulfate and N-oxide abiraterone sulfate. The main metabolite excreted in urine was N-oxide abiraterone sulfate (4.22% of TR). Major components of TR in faeces were unchanged AA (55.3% of TR) and abiraterone (22.3% of TR). Mean recovery of TR in faeces was 87.9%, indicating faeces as primary route of excretion.