Metabolism of 17α-hydroxyprogesterone caproate by hepatic and placental microsomes of human and baboons
Introduction
Preterm delivery (<37 weeks of gestation) is a leading cause of perinatal and neonatal morbidity and mortality. The benefits of 17α-hydroxyprogesterone caproate (17-HPC) in reducing the rate of recurrent preterm delivery in women with history of prior spontaneous preterm birth have been demonstrated in a recent clinical trial [1]. These findings renewed the interest of clinical and basic scientists in investigating the role of natural and synthetic progestins in preventing preterm delivery. However, reported data on the efficacy and clinical benefits of 17-HPC have been controversial [2], [3].
During the last 3 years, the newly formed Obstetric-Fetal Pharmacology Research Units Network of NICHD sponsored investigations of the pharmacokinetics (PK) and pharmacodynamics (PD) of 17-HPC. At our site, University of Texas, the investigations focused on placental transfer and metabolism of 17-HPC as well as validating the nonhuman Papio cynocephalus (baboon) as an animal model for determining the PK and PD of drugs in the pregnant patient. The choice of the baboon is for the following reasons: its 95% DNA homology with humans [4]; anatomical and physiological similarities in fetal development [5]; placental shape, structure and maternal-fetal integration, organization and functions are almost identical with humans [6]. In addition, several CYP450 enzymes and their activities were identified and characterized in primates [7], and only a few interspecies biochemical differences were reported between them and humans [8], [9], [10].
Recent investigations in this laboratory on the in vitro hydrolysis of 17-HPC, radiolabeled in the progesterone [3H] and caproate [14C] revealed that the drug is neither hydrolyzed by human plasma, nor homogenates of human liver, term and preterm placentas [11]. Subsequent investigations on human placental transfer, metabolism and distribution of the radiolabeled 17-α-hydroxy-[3H] progesterone [14C] caproate utilizing dual perfused placental lobule revealed that 17-HPC was transferred to the fetal circuit, metabolized and retained by the placental tissue. The metabolite formed was more polar than the parent compound and was also transferred to the fetal circuit. Both the parent compound and its metabolite retained both [3H] and [14C] confirming its metabolism by a pathway that does not include its hydrolysis by human placental tissue [12]. To the best of our knowledge, data on 17-HPC metabolism via alternative pathway(s) are non-existent.
Human liver is the primary organ responsible for the metabolism of drugs and xenobiotics. However, during pregnancy, the placenta acts as a functional barrier, by virtue of its metabolic enzymes and efflux transporters, thus protecting the fetus from drugs and environmental toxins. Placental metabolic enzymes can contribute up to approximately 10% of the metabolism of a drug, which is in concert with the fetal circulation being approximately 4% of the maternal [13]. It should be noted that metabolite(s) formed by placental tissue should be more accessible to the fetal circulation than those formed by maternal liver.
Therefore, the hypothesis for this investigation is that 17-HPC is metabolized by human and baboon hepatic and placental microsomes.
Section snippets
Chemicals and supplies
All chemicals were purchased from Sigma Chemical Co. (St. Louis, MO, USA) unless otherwise mentioned. Acetonitrile (Optima) and methylene chloride were purchased from Fisher Scientific (Fair Lawn, NJ, USA). 17α-Hydroxy[1,2,6,7-3H]-progesterone [1-14C] caproate (Fig. 1A) was custom synthesized by RTI International (Research Triangle Park, NC, USA). The specific activity of 17-HPC was 26.3 mCi/mmol for [14C] and 52.6 mCi/mmol for [3H].
Human and baboon tissues
A pool of 15 donor human liver microsomes was purchased from
Results
The structure of 17-HPC (Fig. 1A) reflects the position of the radio-nuclides [3H] and [14C]. The retention time for 17-HPC is 34 min (Fig. 1B). The mass spectrum of 17-HPC revealed its molecular ion [M+H] + at m/z 429 and its progesterone fragment at m/z 313 which is a result of a break of the ester bond between the acylcaproate and the hydroxyl- (C-17) of progesterone (Fig. 1C). The acetonitrile in the mobile phase (41 mass units) formed two adducts: one with the parent compound (17-HPC) and
Discussion
The aim of this investigation was to determine whether 17-HPC is metabolized by human and baboon hepatic and placental microsomes. A recent investigation in our laboratory revealed that 17-HPC was not hydrolyzed in vitro by human plasma and homogenates of human livers and placental trophoblast tissue [11]. Moreover, the drug was not hydrolyzed during its transfer across the dually perfused placental lobule but an unknown metabolite of 17-HPC was formed and partially identified [12]. These data
Acknowledgments
The authors greatly appreciate the assistance of the medical staff, the Perinatal Research Division, and the Publication, Grant, & Media Support Office of the Department of Obstetrics and Gynecology, University of Texas Medical Branch, Galveston, Tx. Supported by a grant from the NICHD Obstetric-Fetal Pharmacology Research Units Network grant (OPRU).
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