Absorption: Female patients with a singleton pregnancy received intramuscular doses of 250 mg hydroxyprogesterone caproate for the reduction of preterm birth starting between 16 weeks 0 days and 20 weeks 6 days. All patients had blood drawn daily for 7 days to evaluate pharmacokinetics.
Table 4 Summary of Mean (Standard Deviation) Pharmacokinetics Parameters for Hydroxyprogesterone Caproate
Group (N) | Cmax (ng/mL) | Tmax (days)a | AUC(1 to t)b (ng•hr/mL) |
Group 1 (N=6) | 5.0 (1.5) | 5.5 (2.0 to 7.0) | 571.4 (195.2) |
Group 2 (N=8) | 12.5 (3.9) | 1.0 (0.9 to 1.9) | 1269.6 (285.0) |
Group 3 (N=11) | 12.3 (4.9) | 2.0 (1.0 to 3.0) | 1268.0 (511.6) |
Blood was drawn daily for 7 days (1) starting 24 hours after the first dose between Weeks 16 to 20 (Group 1), (2) after a dose between Weeks 24 to 28 (Group 2), or (3) after a dose between Weeks 32 to 36 (Group 3)
aReported as median (range)
bt = 7 days
For all three groups, peak concentration (Cmax) and area under the curve (AUC(1 to 7 days)) of the mono-hydroxylated metabolites were approximately 3 to 8-fold lower than the respective parameters for the parent drug, hydroxyprogesterone caproate. While di-hydroxylated and tri-hydroxylated metabolites were also detected in human plasma to a lesser extent, no meaningful quantitative results could be derived due to the absence of reference standards for these multiple hydroxylated metabolites. The relative activity and significance of these metabolites are not known.
The elimination half-life of hydroxyprogesterone caproate, as evaluated from 4 patients in the study who reached full-term in their pregnancies, was 16.4 (+3.6) days. The elimination half-life of the mono-hydroxylated metabolites was 19.7 (+6.2) days.
In a single-dose, open-label, randomized, parallel design bioavailability study in 120 healthy post-menopausal women, comparable systemic exposure of hydroxyprogesterone caproate was seen when hydroxyprogesterone caproate injection was dosed intramuscularly (1 mL) in the upper outer quadrant of the gluteus maximus.
Distribution: Hydroxyprogesterone caproate binds extensively to plasma proteins including albumin and corticosteroid binding globulins.
Metabolism:In vitro studies have shown that hydroxyprogesterone caproate can be metabolized by human hepatocytes, both by phase I and phase II reactions. Hydroxyprogesterone caproate undergoes extensive reduction, hydroxylation and conjugation. The conjugated metabolites include sulfated, glucuronidated and acetylated products. In vitro data indicate that the metabolism of hydroxyprogesterone caproate is predominantly mediated by CYP3A4 and CYP3A5. The in vitro data indicate that the caproate group is retained during metabolism of hydroxyprogesterone caproate.
Excretion: Both conjugated metabolites and free steroids are excreted in the urine and feces, with the conjugated metabolites being prominent. Following intramuscular administration to pregnant women at 10 to 12 weeks gestation, approximately 50% of a dose was recovered in the feces and approximately 30% recovered in the urine.
Drug Interactions
Cytochrome P450 (CYP) enzymes: An in vitro inhibition study using human liver microsomes and CYP isoform-selective substrates indicated that hydroxyprogesterone caproate increased the metabolic rate of CYP1A2, CYP2A6, and CYP2B6 by approximately 80%, 150%, and 80%, respectively. However, in another in vitro study using human hepatocytes under conditions where the prototypical inducers or inhibitors caused the anticipated increases or decreases in CYP enzyme activities, hydroxyprogesterone caproate did not induce or inhibit CYP1A2, CYP2A6, or CYP2B6 activity. Overall, the findings indicate that hydroxyprogesterone caproate has minimal potential for CYP1A2, CYP2A6, and CYP2B6 related drug-drug interactions at the clinically relevant concentrations.
In vitro data indicated that therapeutic concentration of hydroxyprogesterone caproate is not likely to inhibit the activity of CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4.