The single-dose pharmacokinetics of plerixafor 0.24 mg/kg were evaluated in patients with NHL and MM following pretreatment with G-CSF (10 micrograms/kg once daily for 4 consecutive days). Plerixafor exhibits linear kinetics between the 0.04 mg/kg to 0.24 mg/kg dose range. The pharmacokinetics of plerixafor was similar across clinical studies in healthy subjects who received plerixafor alone and NHL and MM patients who received plerixafor in combination with G-CSF.
A population pharmacokinetic analysis incorporated plerixafor data from 63 subjects (NHL patients, MM patients, subjects with varying degrees of renal impairment, and healthy subjects) who received a single subcutaneous dose (0.04 mg/kg to 0.24 mg/kg) of plerixafor. A two-compartment disposition model with first order absorption and elimination was found to adequately describe the plerixafor concentration-time profile. Significant relationships between clearance and creatinine clearance (CLCR), as well as between central volume of distribution and body weight were observed. The distribution half-life (t1/2α) was estimated to be 0.3 hours and the terminal population half-life (t1/2β) was 5.3 hours in patients with normal renal function.
The population pharmacokinetic analysis showed that the mg/kg-based dosage results in an increased plerixafor exposure (AUC0-24h) with increasing body weight. In order to compare the pharmacokinetics and pharmacodynamics of plerixafor following 0.24 mg/kg-based and fixed (20 mg) doses, a follow-up trial was conducted in patients with NHL (N=61) who were treated with 0.24 mg/kg or 20 mg of plerixafor. The trial was conducted in patients weighing 70 kg or less. The fixed 20 mg dose showed 1.43-fold higher exposure (AUC0-10h) than the 0.24 mg/kg dose (Table 4). The fixed 20 mg dose also showed numerically higher response rate (5.2% [60% vs 54.8%] based on the local lab data and 11.7% [63.3% vs 51.6%] based on the central lab data) in attaining the target of ≥5 × 106 CD34+ cells/kg than the mg/kg-based dose. However, the median time to reach ≥5 × 106 CD34+ cells/kg was 3 days for both treatment groups, and the safety profile between the groups was similar. Based on these results, further analysis was conducted by FDA reviewers and a body weight of 83 kg was selected as an appropriate cut-off point to transition patients from fixed to weight based dosing.
Table 4: Systemic Exposure (AUC0-10h) Comparisons of Fixed and Weight-Based Regimens
Regimen
| Geometric Mean AUC
|
Fixed 20 mg (n=30)
| 3991.2
|
0.24 mg/kg (n=31)
| 2792.7
|
Ratio (90% CI)
| 1.43 (1.32,1.54)
|
There is limited experience with the 0.24 mg/kg dose of plerixafor in patients weighing above 160 kg. Therefore, the dose should not exceed that of a 160 kg patient (i.e., 40 mg/day if CLCR is greater than 50 mL/min and 27 mg/day if CLCR is less than or equal to 50 mL/min) [see Dosage and Administration (2.1, 2.3)].
Absorption
Peak plasma concentrations occurred at approximately 30 to 60 minutes after a subcutaneous dose.
Distribution
Plerixafor is bound to human plasma proteins up to 58%. The apparent volume of distribution of plerixafor in humans is 0.3 L/kg demonstrating that plerixafor is largely confined to, but not limited to, the extravascular fluid space.
Metabolism
The metabolism of plerixafor was evaluated with in vitro assays. Plerixafor is not metabolized as shown in assays using human liver microsomes or human primary hepatocytes and does not exhibit inhibitory activity in vitro towards the major drug metabolizing cytochrome P450 enzymes (1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4/5). In in vitro studies with human hepatocytes, plerixafor does not induce CYP1A2, CYP2B6, or CYP3A4 enzymes. These findings suggest that plerixafor has a low potential for involvement in cytochrome P450-dependent drug-drug interactions.
Elimination
The major route of elimination of plerixafor is urinary. Following a 0.24 mg/kg dose in healthy volunteers with normal renal function, approximately 70% of the dose was excreted in the urine as the parent drug during the first 24 hours following administration. In studies with healthy subjects and patients, the terminal half-life in plasma ranges between 3 and 5 hours. At concentrations similar to what are seen clinically, plerixafor did not act as a substrate or inhibitor of P-glycoprotein in an in vitro study with MDCKII and MDCKII-MDR1 cell models.
Special Populations
Renal impairment
Following a single 0.24 mg/kg subcutaneous dose, plerixafor clearance was reduced in subjects with varying degrees of renal impairment and was positively correlated with CLCR. The mean AUC0 24h of plerixafor in subjects with mild (CLCR 51 to 80 mL/min), moderate (CLCR 31 to 50 mL/min), and severe (CLCR <31 mL/min) renal impairment was 7%, 32%, and 39% higher than healthy subjects with normal renal function, respectively. Renal impairment had no effect on Cmax. A population pharmacokinetic analysis indicated an increased exposure (AUC0-24h) in patients with moderate and severe renal impairment compared to patients with CLCR >50 mL/min. These results support a dose reduction of one-third in patients with moderate to severe renal impairment (CLCR ≤50 mL/min) in order to match the exposure in patients with normal renal function. The population pharmacokinetic analysis showed that the mg/kg-based dosage results in an increased plerixafor exposure (AUC0-24h) with increasing body weight; therefore, if CLCR is ≤50 mL/min the dose should not exceed 27 mg/day [see Dosage and Administration (2.3)].
Since plerixafor is primarily eliminated by the kidneys, coadministration of plerixafor with drugs that reduce renal function or compete for active tubular secretion may increase serum concentrations of plerixafor or the coadministered drug. The effects of coadministration of plerixafor with other drugs that are renally eliminated or are known to affect renal function have not been evaluated.
Race
Clinical data show similar plerixafor pharmacokinetics for Caucasians and African Americans, and the effect of other racial/ethnic groups has not been studied.
Gender
Clinical data show no effect of gender on plerixafor pharmacokinetics.
Age
Clinical data show no effect of age on plerixafor pharmacokinetics.