An examination of whether human placental perfusion allows accurate prediction of placental drug transport: Studies with diazepam

Abstract

Introduction: Presently, no well-validated predictive tools are available for human placental transfer. We studied the transplacental passage of diazepam (DZP) in a recirculating dual human placental perfusion and compared the data with in vivo clinical data from the literature. Methods: Term placentas from healthy mothers without medication were used. The dual, recirculating perfusion technique was used. DZP (2 μg/ml, n=4; 200 ng/ml, n=3) and the reference compound antipyrine (100 μg/ml) were added into the maternal circulation simultaneously. The disappearance of drugs from the maternal circulation and appearance into the fetal circulation were followed every 15 min for 2 h. Results: DZP was detectable in the fetal circulation within 15 min in all of the perfusions indicating rapid transfer. DZP concentrations in the maternal circulation were higher than in the fetal circulation throughout the perfusion with both initial concentrations. At the end of the perfusion, the feto-maternal ratio was 0.48±0.11 (mean±S.D.) and the transfer from the maternal to the fetal compartment 18.4±3.6% with 2 μg/ml of DZP and 0.55±0.10 and 20.5±3.1% with 200 ng/ml of DZP, respectively. DZP concentrations in the perfused area of the placenta were in average 2 times higher than in the maternal perfusate and 3.6 times higher than in the fetal perfusate. Total recovery of DZP from samples, perfusion fluid, and perfused tissue was 37.6±21%. Discussion: Since animal studies in vivo do not accurately predict human placental transfer and it is problematic to study placental transfer of drugs in humans in vivo, the present human placental perfusion system could serve as one part of a test battery for fetotoxicity. However, although our earlier studies and those from the literature indicate a good correlation between in vivo and placental perfusion data, the present study shows this is not the case for all drugs.

Introduction

Exposure to prescription and nonprescription drugs during pregnancy is common Koren et al., 1998, Lacroix et al., 2000, Olesen et al., 1999. Nearly all drugs cross the placenta to some extent Pacifici & Nottoli, 1995, Schneider & Malek, 1995. The most common mechanism of placental transfer is passive diffusion but some drugs are also substrates for placental transporters (Ganapathy, Prasad, Ganapathy, & Leibach, 2000). Drugs may therefore affect the placental transport of endogenous substrates through these transporters (Ganapathy et al., 2000).

For the development of safe and effective pharmacotherapy, it is important to have knowledge of the pharmacokinetics of drugs in the feto-placental unit. However, thus far, no well-validated predictive tools for human placental transfer have been available (Ala-Kokko, Myllynen, & Vähäkangas, 2000). Traditionally, clinical trials are not conducted in pregnant women for ethical reasons. For the same reason, it is also clearly impossible to study human transplacental passage of possibly harmful agents in vivo. Because adverse effects are often dose dependent (Ala-Kokko et al., 2000), knowledge of placental transfer of drugs is important for risk assessment. Although animal studies are valuable in the assessment of many aspects of teratogenicity Claudio et al., 2000, Koren et al., 1998, the placental structure shows great interspecies variation (Leiser & Kaufmann, 1994) making it unwise to extrapolate placental transfer data from animal studies to humans (Ala-Kokko et al., 2000).

During the past decades, several in vitro methods have been developed for the study of placental transfer using human and animal tissues Ala-Kokko et al., 2000, Sastry, 1999. One of these methods is the placental perfusion of a single human cotyledon (Panigel, 1962). There have been several modifications of this method Ala-Kokko et al., 2000, Miller et al., 1989, Pienimäki et al., 1995, Schneider et al., 1972. We use a dual recirculating system in which both maternal and fetal sides are perfused Ala-Kokko et al., 1995, Ala-Kokko et al., 1997, Lampela et al., 1999, Olivero et al., 1999, Pienimäki et al., 1995, Pienimäki et al., 1997. Because the placenta is collected after birth, placental perfusion can be used to examine the transfer of potentially toxic substances without concern for the maternal or fetal safety. In addition to transfer, placental perfusion allows evaluation of a wide range of other variables at the same time, including placental metabolism, production and release of hormones and enzymes, transport of nutrients and waste products, etc. (Slikker & Miller, 1994). So far, the human placental perfusion method has been used to study placental transfer of several nutrients, hormones, vitamins, therapeutic agents, abused drugs, and toxic chemicals Ala-Kokko et al., 2000, Bourget et al., 1995, Sastry, 1999.

It has been proposed that human placental perfusion has the potential to be used as method for preclinical evaluation of transplacental passage and placental metabolism of new drugs Ala-Kokko et al., 2000, Pienimäki et al., 1997, Tuntland et al., 1999. Several examples from the literature suggest a similar pharmacokinetic profile in vivo and in the placental perfusion model Ala-Kokko et al., 2000, Omarini et al., 1992, Tuntland et al., 1999. However, placental perfusion needs further systematic characterization. Diazepam (DZP) has been in clinical use, including during pregnancy, for many years and there is abundant clinical information available concerning its placental passage (Iqbal, Sobhan, Aftab, & Mahmud, 2002). The drug therefore provides a suitable template for evaluating placental passage in the perfusion model. Therefore, in the present study, we have examined the transplacental passage of DZP in human placental perfusion and compared the data with in vivo clinical data from the literature.