CLINICAL PHARMACOLOGY
Mechanism of Action
Abarelix exerts its pharmacological action by directly suppressing luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion and thereby reducing
the secretion of testosterone by the testes. Due to the direct inhibition of
the secretion of LH by abarelix, there is no initial increase in serum testosterone
concentrations. Saturation binding studies revealed that [125I]-abarelix
has a very high affinity (KD = 0.1 nM) for the rat pituitary LHRH receptor.
Pharmacokinetics
A single dose (100 mg IM) of Plenaxis™ was given to 14 healthy male volunteers
52 to 75 years of age, with body weight of 61.6 to 110.5 kg, and the pharmacokinetic
information is provided in Table 1
Table 1: Mean ± SD Pharmacokinetic Parameter Values
of 100 mg of Plenaxis™ Following a Single IM Injection (n = 14)
Cmax (ng/mL) |
Tmax (days) |
AUC0-∞ (ng •day/mL) |
CL/F (L/day) |
t½ (days) |
43.4 ± 32.3 |
3.0 ± 2.9 |
500 ± 96 |
208 ± 48 |
13.2 ± 3.2 |
Absorption
Following IM administration of 100 mg of Plenaxis™ , abarelix is absorbed
slowly with a mean peak concentration of 43.4 ng/mL observed approximately 3
days after the injection.
Distribution
The apparent volume of distribution during the terminal phase determined after
IM administration of Plenaxis™ was 4040 ± 1607 liters, implying
that abarelix probably distributes extensively within the body.
Metabolism
In vitro hepatocyte (rat, monkey, human) studies and in vivo studies
in rats and monkeys showed that the major metabolites of abarelix were formed
via hydrolysis of peptide bonds. No significant oxidative or conjugated metabolites
of abarelix were found either in vitro or in vivo. There is no evidence
of cytochrome P-450 involvement in the metabolism of abarelix.
Excretion
In humans, approximately 13% of unchanged abarelix was recovered in urine after
a 15 μg/kg IM injection; there were no detectable metabolites in urine. Renal
clearance of abarelix was 14.4 L/day (or 10 mL/min) after administration of
100 mg Plenaxis™ .
Pharmacodynamics
Effects of Plenaxis™ on Serum Testosterone: The effectiveness
of Plenaxis™ in suppressing serum testosterone was studied in two randomized,
open-label, activecomparator trials. Patients were not those with advanced symptomatic
prostate cancer. They were randomized in a 2:1 ratio to Plenaxis™ 100
mg IM versus LHRH agonist (Study 1) or to Plenaxis™ versus LHRH agonist
+ nonsteroidal antiandrogen (Study 2). Plenaxis™ was administered IM on
Days 1, 15, 29 (Week 4), then every 4 weeks thereafter for at least 6 months
(24 weeks). LHRH agonist and nonsteroidal antiandrogen were administered in
standard fashion. After completing 6 months of treatment, patients could continue
randomized treatment for an additional 6 months.
Avoidance of testosterone surge: In both studies combined, 100% (348/348)
of Plenaxis™ patients and 16% (28/172) of comparator patients avoided
a testosterone surge.
Attainment of medical castration: The percentage of patients who attained
serum testosterone concentration ≤ 50 ng/dL on Study Days 2, 8, 15 and 29
are summarized in the table below:
Table 2: Percentage of patients who attained medical castration
(serum testosterone concentration ≤ 50 ng/dL) in Studies 1 and 2.
Day |
Plenaxis™ |
Total N |
% Castrate |
2 |
339 |
24 |
4 |
333 |
56 |
8 |
348 |
70 |
15 |
347 |
73 |
29 |
347 |
94 |
Attainment and maintenance of medical castration: Successful response
was defined as attainment of medical castration on Day 29 and maintenance through
Day 85 (where no two consecutive serum testosterone concentrations between Days
29 and 85 were greater than 50 ng/dL). In Study 1, 92% on Plenaxis™ patients
responded and 96% of LHRH agonist patients responded. In Study 2, 93% of Plenaxis™
patients and 95% of LHRH agonist + nonsteroidal antiandrogen patients responded.
However, when failure was defined as any observed serum testosterone > 50
ng/Dl (including transient elevations) just prior to dosing on Day 29 and every
28 days thereafter, effectiveness of testosterone suppression decreased over
time. Results of this analysis are summarized in Table 3.
Table 3: Percentage of patients who attained and maintained
medical castration; [no serum testosterone > 50 ng/dL just prior to dosing
on Day 29 and every 28 days thereafter]
Day |
Study 1 Plenaxis™ |
N |
Study 2 Plenaxis™ |
N |
85 |
84% |
176 |
92% |
164 |
169 |
75% |
166 |
87% |
155 |
365 |
62% |
93 |
71% |
86 |
Effects of Plenaxis™ on Cardiac Electrophysiology: In a single,
active-controlled, clinical study comparing Plenaxis™ to LHRH agonist
+ nonsteroidal antiandrogen, periodic electrocardiograms were performed. Both
therapies prolonged the mean Fridericia-corrected QT interval by > 10 msec
from baseline. In approximately 20% of patients in both groups, there were either
changes from baseline QTc of > 30 msec, or end-of-treatment QTc values exceeding
450 msec. Similar results were observed in 2 other Phase 3 studies with Plenaxis™
and the active-control treatments. It is unclear whether these changes were
directly related to study drugs, to androgen deprivation therapy, or to other
variables.
Special Populations
Race
Data from Hispanics, Blacks and Caucasians demonstrated that race appeared
to have no influence on the pharmacokinetics of Plenaxis™ .
Renal and Hepatic Insufficiency
The pharmacokinetics of Plenaxis™ in hepatically and/or renally impaired
patients have not been determined.
Pediatric Use
There have been no studies of Plenaxis™ in pediatric patients.
Clinical Studies
One study of Plenaxis™ was conducted in 81 men with advanced symptomatic prostate cancer who were at risk for clinical exacerbation (“clinical
flare”) if treated with an LHRH agonist. The objective of this open-label,
multicenter, uncontrolled, single-arm study was to demonstrate that such patients
could avoid orchiectomy through at least 12 weeks of treatment. In this trial,
treatment was to be given for at least 6 months with the option to continue
treatment in an extension trial.
Of the 81 patients who enrolled, 9 patients from one site were excluded from
the efficacy analysis due to inadequate documentation by the study investigator.
The specific reasons given for enrollment of the 72 patients were: bone pain
from prostate cancer skeletal metastases (n = 31); an enlarged prostate gland
or pelvic mass causing bladder neck outlet obstruction (n = 25); bilateral retroperitoneal
adenopathy with ureteral obstruction (n = 9); impending neurological compromise
from spinal, spinal cord, or epidural metastases (n = 6); or other (n = 1).
The median age was 73 years, range 40 to 94 years. There were 62 Caucasians,
6 African Americans and 4 Hispanics.
Plenaxis™ 100 mg was administered via IM injection on Days 1, 15 and
29, then every 4 weeks thereafter. Twelve patients discontinued prior to Day
169 for the following reasons: adverse event (n=2), voluntary withdrawal (n=3),
death (n=4), and “other” (n=3). Sixty patients were treated for
at least 24 weeks; in the extension phase, 33 patients for at least 48 weeks
and 15 patients for at least 96 weeks. None (0%) of the 72 patients required
orchiectomy while being treated with Plenaxis™ , including the extension
phase (median combined duration of therapy was 40 weeks). However, 2 patients
were withdrawn before week 12 for treatment-related adverse events (immediate-onset systemic allergic reactions consisting of urticaria, and urticaria and pruritis,
respectively) and received alternate therapy. In this trial, medical castration
(defined as serum total testosterone concentration ≤ 50 ng/dL) was achieved
in 57 of the 72 patients (79%) by Day 8, and by 68 of 71 patients (96%) by Week
4. Although the study was not designed to assess specific clinical outcomes,
the following were observed:
- None (0) of 8 patients with vertebral or epidural metastases and without
neurological symptoms developed neurological symptoms.
- Ten of 13 patients with bladder outlet obstruction and a bladder drainage
catheter had the catheter removed by 12 weeks.
- Eleven of 15 patients with pain due to skeletal metastases were able to
reduce the potency, dose and/or frequency of narcotic analgesia at 12 weeks.