CLINICAL PHARMACOLOGY
Mechanism Of Action
Abiraterone acetate (YONSA) is converted in vivo to abiraterone, an androgen biosynthesis inhibitor, that inhibits 17 α-hydroxylase/C17, 20-lyase (CYP17). This enzyme is expressed in testicular, adrenal, and prostatic tumor tissues and is required for androgen biosynthesis.
CYP17 catalyzes two sequential reactions: 1) the conversion of pregnenolone and progesterone to their 17αhydroxy
derivatives by 17α-hydroxylase activity and 2) the subsequent formation of dehydroepiandrosterone (DHEA) and androstenedione, respectively, by C17, 20 lyase activity. DHEA and androstenedione are androgens and are precursors of testosterone. Inhibition of CYP17 by abiraterone can also result in increased mineralocorticoid production by the adrenals [see WARNINGS AND PRECAUTIONS].
Androgen sensitive prostatic carcinoma responds to treatment that decreases androgen levels. Androgen deprivation therapies, such as treatment with GnRH agonists or orchiectomy, decrease androgen production in the testes but do not affect androgen production by the adrenals or in the tumor.
Abiraterone acetate decreased serum testosterone and other androgens in patients in the placebo-controlled phase 3 clinical trial. It is not necessary to monitor the effect of YONSA on serum testosterone levels.
Changes in serum prostate specific antigen (PSA) levels may be observed but have not been shown to correlate with clinical benefit in individual patients.
Pharmacodynamics
In a clinical study in patients with metastatic CRPC who were treated with YONSA 500 mg once daily and methylprednisolone 4 mg twice daily for 84 days, the average serum testosterone level ± standard deviation (SD) on days 9 and 10 of treatment was 0.33 ± 0.09 ng/dL.
Pharmacokinetics
Following administration of abiraterone acetate, the pharmacokinetics of abiraterone and abiraterone acetate have been studied in healthy volunteers and in patients with metastatic CRPC. In vivo, abiraterone acetate is converted to abiraterone. In clinical studies of other abiraterone acetate formulations, abiraterone acetate plasma concentrations were below detectable levels (< 0.2 ng/mL) in > 99% of the analyzed samples.
Geometric mean ±SD abiraterone Cmax was 73 ± 44 ng/mL and AUCINF was 373 ± 249 ng·hr/mL following a single dose of YONSA 500 mg in overnight fasted healthy volunteers. Dose proportionality was observed in single doses of YONSA in a range of 125 mg to 625 mg.
Absorption
Following oral administration of YONSA to healthy volunteers and patients with metastatic CRPC, the mean time to reach maximum plasma abiraterone concentrations is approximately 2 hours.
Effect of Food
Abiraterone Cmax was approximately 6.5-fold higher and AUCINF was 4.4-fold higher when a single dose of YONSA 500 mg was administered with a high-fat meal (56-60% fat, 900-1,000 calories) compared to overnight fasting in healthy volunteers.
Other formulations of abiraterone acetate may differ in their food effects and dose. This may impact the ability to take other abiraterone formulations with food. YONSA can be taken with or without food [see DOSAGE AND ADMINISTRATION].
Distribution And Protein Binding
Abiraterone is highly bound (>99%) to the human plasma proteins, albumin and alpha-1 acid glycoprotein. The apparent steady-state volume of distribution (mean ± SD) is 19,669 ± 13,358 L. In vitro studies show that at clinically relevant concentrations, abiraterone acetate and abiraterone are not substrates of P-glycoprotein (Pgp)
and that abiraterone acetate is an inhibitor of P-gp.
Metabolism
Following oral administration of 14C-abiraterone acetate as capsules, abiraterone acetate is hydrolyzed to abiraterone (active metabolite). The conversion is likely through esterase activity (the esterases have not been identified) and is not CYP mediated. The two main circulating metabolites of abiraterone in human plasma are abiraterone sulphate (inactive) and N-oxide abiraterone sulphate (inactive), which account for about 43% of exposure each. CYP3A4 and SULT2A1 are the enzymes involved in the formation of N-oxide abiraterone sulphate and SULT2A1 is involved in the formation of abiraterone sulphate.
Excretion
In patients with metastatic CRPC, the mean terminal half-life of abiraterone in plasma (mean ± SD) is 12 ± 5 hours. Following oral administration of 14C-abiraterone acetate, approximately 88% of the radioactive dose is recovered in feces and approximately 5% in urine. The major compounds present in feces are unchanged abiraterone acetate and abiraterone (approximately 55% and 22% of the administered dose, respectively).
Patients With Hepatic Impairment
The pharmacokinetics of abiraterone was examined in subjects with baseline mild (N=8) or moderate (N=8) hepatic impairment (Child-Pugh Class A and B, respectively) and in 8 healthy control subjects with normal hepatic function. Systemic exposure to abiraterone after a single oral 1,000 mg dose of another abiraterone acetate product given under fasting conditions increased approximately 1.1-fold and 3.6-fold in subjects with mild and moderate baseline hepatic impairment, respectively. The mean half-life of abiraterone is prolonged to approximately 18 hours in subjects with mild hepatic impairment and to approximately 19 hours in subjects with moderate hepatic impairment.
In another trial, the pharmacokinetics of abiraterone were examined in subjects with baseline severe (N=8) hepatic impairment (Child-Pugh Class C) and in 8 healthy control subjects with normal hepatic function. They systemic exposure (AUC) of abiraterone increased by approximately 7-fold in subjects with severe baseline hepatic impairment compared to subjects with normal hepatic function. In addition, the mean protein binding was found to be lower in the severe hepatic impairment group compared to the normal hepatic function group, which resulted in a two-fold increase in the fraction of free drug in patients with severe hepatic impairment [see DOSAGE AND ADMINISTRATION and Use In Specific Populations].
Patients With Renal Impairment
The pharmacokinetics of abiraterone were examined in patients with end-stage renal disease (ESRD) on a stable hemodialysis schedule (N=8) and in matched control subjects with normal renal function (N=8). In the ESRD cohort of the trial, a single 1,000 mg dose of another abiraterone acetate product was given under fasting conditions 1 hour after dialysis, and samples for pharmacokinetic analysis were collected up to 96 hours post dose. Systemic exposure to abiraterone after a single oral 1,000 mg dose of another abiraterone acetate product did not increase in subjects with end-stage renal disease on dialysis, compared to subjects with normal renal function [see Use In Specific Populations].
Drug Interactions
In vitro studies with human hepatic microsomes showed that abiraterone has the potential to inhibit CYP1A2, CYP2D6 and CYP2C8 and to a lesser extent CYP2C9, CYP2C19 and CYP3A4/5.
In an in vivo drug-drug interaction trial, the Cmax and AUC of dextromethorphan (CYP2D6 substrate) were increased 2.8-and 2.9-fold, respectively when dextromethorphan 30 mg was given with 1,000 mg daily of
another abiraterone acetate product (plus a different corticosteroid twice daily). The AUC for dextrorphan, the active metabolite of dextromethorphan, increased approximately 1.3 fold [see DRUG INTERACTIONS].
In a clinical study to determine the effects of 1,000 mg daily of another abiraterone acetate product (plus a different corticosteroid twice daily) on a single 100 mg dose of the CYP1A2 substrate theophylline, no increase in systemic exposure of theophylline was observed.
Abiraterone is a substrate of CYP3A4, in vitro. In a clinical pharmacokinetic interaction study of healthy subjects, pretreated with a strong CYP3A4 inducer (rifampin, 600 mg daily for 6 days) followed by a single dose of 1,000 mg of another abiraterone acetate product, the mean plasma AUCINF of abiraterone was decreased by 55% [see DRUG INTERACTIONS].
In a separate clinical pharmacokinetic interaction study of healthy subjects, co-administration of ketoconazole, a strong inhibitor of CYP3A4, had no clinically meaningful effect on the pharmacokinetics of abiraterone [see DRUG INTERACTIONS].
In a CYP2C8 drug-drug interaction trial in healthy subjects, the AUC of pioglitazone was increased by 46% when pioglitazone was given together with a single dose of 1,000 mg of another abiraterone acetate product
[see DRUG INTERACTIONS].
In vitro, abiraterone and its major metabolites were shown to inhibit the hepatic uptake transporter OATP1B1. There are no clinical data available to confirm transporter based
QT Prolongation
In a multi-center, open-label, single-arm trial, 33 patients with metastatic CRPC received a dose of 1,000 mg once daily orally of another abiraterone acetate product at least 1 hour before or 2 hours after a meal in combination with a different corticosteroid orally twice daily. Assessments up to Cycle 2 Day 2 showed no large changes in the QTc interval (i.e., >20 ms) from baseline. However, small increases in the QTc interval (i.e., <10 ms) due to abiraterone acetate cannot be excluded due to study design limitations.
Animal Toxicology And/Or Pharmacology
A dose-dependent increase in cataracts was observed in rats at 26 weeks starting at ≥ 50 mg/kg/day (similar to the human clinical exposure based on AUC). In the 39-week monkey study, no cataracts were observed at higher doses (2 times greater than the clinical exposure based on AUC).
Clinical Studies
The efficacy and safety of abiraterone acetate in patients with metastatic castration-resistant prostate cancer (CRPC) that has progressed on androgen deprivation therapy was demonstrated in two randomized, placebo-controlled, multicenter phase 3 clinical trials. Patients with prior ketoconazole treatment for prostate cancer and a history of adrenal gland or pituitary disorders were excluded from these trials. Concurrent use of spironolactone was not allowed during the study period.
Study 1
Patients with metastatic CRPC who had received prior docetaxel chemotherapy:
A total of 1195 patients were randomized 2:1 to receive either abiraterone acetate orally at a dose equivalent to 500 mg of YONSA once daily in combination with a different corticosteroid orally twice daily (N=797) or placebo once daily plus a different corticosteroid orally twice daily (N=398). Patients randomized to either arm were to continue treatment until disease progression (defined as a 25% increase in PSA over the patient’s baseline/nadir together with protocol-defined radiographic progression and symptomatic or clinical progression), initiation of new treatment, unacceptable toxicity or withdrawal.
The following patient demographics and baseline disease characteristics were balanced between the treatment arms. The median age was 69 years (range 39-95) and the racial distribution was 93.3% Caucasian, 3.6% Black, 1.7% Asian, and 1.6% Other. Eighty-nine percent of patients enrolled had an ECOG performance status score of 0-1 and 45% had a Brief Pain Inventory-Short Form score of ≥ 4 (patient’s reported worst pain over the previous 24 hours). Ninety percent of patients had metastases in bone and 30% had visceral involvement. Seventy percent of patients had radiographic evidence of disease progression and 30% had PSA-only progression. Seventy percent of patients had previously received one cytotoxic chemotherapy regimen and 30% received two regimens.
The protocol pre-specified interim analysis was conducted after 552 deaths and showed a statistically significant improvement in overall survival in patients treated with abiraterone acetate compared to patients in the placebo arm (Table 5 and Figure 1). An updated survival analysis was conducted when 775 deaths (97% of the planned
number of deaths for final analysis) were observed. Results from this analysis were consistent with those from the interim analysis (Table 5).
Table 5: Overall Survival of Patients Treated with Either Abiraterone Acetate or Placebo in Combination with Corticosteroid in Study 1 (Intent-to-Treat Analysis)
|
Abiraterone Acetate with Corticosteroid (N=797) |
Placebo with Corticosteroid (N=398) |
Primary Survival Analysis |
|
|
Deaths (%) |
333 (42%) |
219 (55%) |
Median survival (months) (95% CI) |
14.8 (14.1, 15.4) |
10.9 (10.2, 12.0) |
p-value1 |
<0.0001 |
Hazard ratio (95% CI)2 |
0.646 (0.543, 0.768) |
Updated Survival Analysis |
|
|
Deaths (%) |
501 (63%) |
274 (69%) |
Median survival (months) (95% CI) |
15.8 (14.8, 17.0) |
11.2 (10.4, 13.1) |
Hazard ratio (95% CI)2 |
0.740 (0.638, 0.859) |
1 p-value is derived from a log-rank test stratified by ECOG performance status score (0-1 vs. 2), pain score (absent vs. present), number of prior chemotherapy regimens (1 vs. 2), and type of disease progression (PSA only vs. radiographic)
2 Hazard Ratio is derived from a stratified proportional hazards model. Hazard ration <1 favors abiraterone acetate |
Figure 1: Kaplan-Meier Overall Survival Curves in Study 1 (Intent-to-Treat Analysis)
Study 2
Patients with metastatic CRPC who had not received prior cytotoxic chemotherapy
In Study 2, 1088 patients were randomized 1:1 to receive either abiraterone acetate at a dose equivalent to 500 mg of YONSA once daily (N=546) or placebo once daily (N=542). Both arms were also given a different corticosteroid twice daily. Patients continued treatment until radiographic or clinical (cytotoxic chemotherapy, radiation or surgical treatment for cancer, pain requiring chronic opioids, or ECOG performance status decline to 3 or more) disease progression, unacceptable toxicity or withdrawal. Patients with moderate or severe pain, opiate use for cancer pain, or visceral organ metastases were excluded.
Patient demographics were balanced between the treatment arms. The median age was 70 years. The racial distribution of patients treated with abiraterone acetate was 95% Caucasian, 2.8% Black, 0.7% Asian and 1.1% Other. The ECOG performance status was 0 for 76% of patients, and 1 for 24% of patients. Co-primary efficacy endpoints were overall survival and radiographic progression-free survival (rPFS). Baseline pain assessment was 0-1 (asymptomatic) in 66% of patients and 2-3 (mildly symptomatic) in 26% of patients as defined by the Brief Pain Inventory-Short Form (worst pain over the last 24 hours).
Radiographic progression-free survival was assessed with the use of sequential imaging studies and was defined by bone scan identification of 2 or more new bone lesions with confirmation (Prostate Cancer Working Group 2 criteria) and/or modified Response Evaluation Criteria In Solid Tumors (RECIST) criteria for progression of soft tissue lesions. Analysis of rPFS utilized centrally-reviewed radiographic assessment of progression.
The planned final analysis for OS, conducted after 741 deaths (median follow up of 49 months) demonstrated a statistically significant OS improvement in patients treated with abiraterone acetate compared to those treated with placebo (Table 6 and Figure 2). Sixty-five percent of patients on the abiraterone acetate arm and 78% of patients on the placebo arm used subsequent therapies that may prolong OS in metastatic CRPC. Abiraterone acetate was used as a subsequent therapy in 13% of patients on the abiraterone acetate arm and 44% of patients on the placebo arm.
Table 6: Overall Survival of Patients Treated with Either Abiraterone Acetate or Placebo in Combination with Corticosteroid in Study 2 (Intent-to-Treat Analysis)
Overall Survival |
Abiraterone Acetate with Corticosteroid (N=546) |
Placebo with Corticosteroid (N=542) |
Deaths |
354 (65%) |
387 (71%) |
Median survival (months) (95% CI) |
34.7 (3.7, 36.8) |
30.3 (28.7, 33.3) |
p-value1 |
0.0033 |
Hazard ratio2 (95% CI) |
0.81 (0.70, 0.93) |
1 P-value is derived from a log-rank test stratified by ECOG performance status score (0 vs. 1).
2 Hazard Ratio is derived from a stratified proportional hazards model. Hazard ratio <1 favors Abiraterone Acetate |
Figure 2 – Kaplan Meier Overall Survival Curves in Study 2
At the pre-specified rPFS analysis, 150 (28%) patients treated with abiraterone acetate and 251 (46%) patients treated with placebo had radiographic progression. A significant difference in rPFS between treatment groups was observed (Table 7 and Figure 3).
Table 7: Radiographic Progression-free Survival of Patients Treated with Either Abiraterone Acetate or Placebo in Combination with Corticosteroid in Study 2 (Intent-to-Treat Analysis)
Radiographic Progression-free Survival |
Abiraterone Acetate with Corticosteroid (N=546) |
Placebo with Corticosteroid (N=542) |
Progression or death |
150 (28%) |
251 (46%) |
Median rPFS (months) (95% CI) |
NR (11.66, NR) |
8.28 (8.12, 8.54) |
p-value1 |
<0.0001 |
Hazard ratio2 (95% CI) |
0.425 (0.347, 0.522) |
NR= Not Reached
1 p-value is derived from a log-rank test stratified by ECOG performance status score (0 vs. 1).
2 Hazard Ratio is derived from a stratified proportional hazards model. Hazard ration <1 favors abiraterone acetate |
Figure 3 – Kaplan Meier Curves of Radiographic Progression-free Survival in Study 2 (Intent-to-Treat
Analysis)
The primary efficacy analyses are supported by the following prospectively defined endpoints. The median time to initiation of cytotoxic chemotherapy was 25.2 months for patients in the abiraterone acetate arm and
16.8 months for patients in the placebo arm (HR=0.580; 95% CI: [0.487, 0.691], p<0.0001).
The median time to opiate use for prostate cancer pain was not reached for patients receiving abiraterone acetate and was 23.7 months for patients receiving placebo (HR=0.686; 95% CI: [0.566, 0.833], p=0.0001). The time to opiate use result was supported by a delay in patient reported pain progression favoring the abiraterone acetate arm.