ABCB1 Gene Variants and P-Glycoprotein-Mediated Efflux of Methodone in Human Placenta
Principal Investigator: Sherif Abdel-Rahman, PhD
Substance abuse during pregnancy is a major public health problem. More than 2.5 million women are pregnant in the US each year, and the prevalence of opiate abuse in this group ranges from 2% to as high as 21%. For the pregnant opiate addict, methadone maintenance therapy has been the standard of care for many decades. However, development of neonatal abstinence syndrome (NAS) is a major problem affecting newborns of mothers in methadone maintenance programs. To date, prescribing physicians are faced with the challenge of balancing benefits of methadone therapy (prevention of repeated intoxication/withdrawal and drug seeking behavior) with risks to the fetus (NAS). There is a lack of correlation between maternal methadone dose and incidence and severity of NAS, which strongly suggests that NAS correlates with methadone concentration in the fetal—and not the maternal—circulation.
Recently, our group demonstrated that the placental efflux transporter P-glycoprotein (P-gp), encoded by the multidrug resistance gene ABCB1(MDR1), actively regulates the extent of methadone transfer across the placenta and consequently affects its concentration in the fetal circulation. The ABCB1 gene is highly polymorphic, with more than 250 single nucleotide polymorphisms (SNPs) within its genomic region. These SNPs (commonly inherited genomic sequences variations) are arrayed as combinations (of many SNPs) in the genome forming specific haplotypes. These haplotypes could alter the level, structure, and function of P-gp, thus ultimately modifying the efflux activity in the placenta and its permeability to methadone. The relationship between ABCB1 haplotypes and P-gp function in human placenta is currently unknown.
The goal of this investigation is to generate preliminary data that will allow us to comprehensively characterize through an R01 project the relationship between ABCB1 haplotypes and P-gp function in the efflux of methadone from the human placenta. To achieve this goal, we will test the hypothesis that specific ABCB1 haplotypes (specific SNPs combinations) alter the efflux of methadone from the human placenta by affecting ABCB1 transcriptional processes and/or its encoded P-gp protein expression levels and activity.
To test our hypothesis, in Aim 1, we will (a) determine whole-gene, high-resolution haplotype structures encompassing the reported common SNPs in the ABCB1 gene and (b) determine the ABCB1 haplotypes of placentas obtained from uncomplicated pregnancies.
In Aim 2, we will characterize in human placentas the relationship between ABCB1 haplotypes and (a) ABCB1 transcription (ie, ABCB1 transcript levels and nature of splice variants) and (b) P-gp protein expression levels. In Aim 3, we will characterize the relationship between ABCB1 haplotypes and P-gp activity, determined by ATP-dependent uptake of [3H]-methadone by inside-out vesicles of placental apical membranes.
The data generated from our study will be used as preliminary data to respond to the critique of a scored R01 proposal that we are currently revising for resubmission. In addition, the data will be germane to larger translational studies already proposed in another submitted R01 project that focuses on elucidating the relationship between placental biodisposition of methadone and incidence and intensity of NAS in newborns of methadone-treated pregnant opiate addicts.
The information generated from our research program will bridge an important gap in knowledge by leading to a better understanding of the genetic factors regulating placental P-gp expression and activity, which, in turn, regulate the concentration of methadone (and other P-gp substrates) in the fetal circulation. The knowledge gained should also enhance our understanding of the genetic factors involved in incidence and intensity of NAS in newborn of women treated with methadone during pregnancy. In the near future, this could lead to the development of individualized treatment approaches for pregnant patients that minimize fetal and neonatal complications.