Similar to cocaine, opiate cross the placenta and cause intrauterine
growth retardation (IUGR) and preterm deliveries. Methadone is
the standard therapy for pregnant opioid-dependent women [84].
The positive effects of methadone are an increase in birth weight
and prolongation of gestation [85,86]. Because co-consumption of
methadone with other drugs such as cocaine and heroin is frequent,
additional drugs may influence the placental transfer of methadone
and other substances by different mechanisms. In case of inhibition of
the P-glycoprotein (P-gp) function by other drugs, the placental barrier
may disrupt, and P-gp substrates may increasingly transfer to fetal
circulation [87,88]. The P-gp which is expressed in the brush-border
of the placental syncytiotrophoblast and this syncytial layer is floating
in the maternal blood. The P-gp is an efflux transporter meaning the
protection of the placenta and the fetus against many drugs [87,88].
In a second perfusion study [89], the effect of the combined cocaine
plus methadone on the placental function was investigated. All
tested compounds were added to the maternal circulation. Under the
conditions of the control experiments or the presence of methadone
alone similar values were observed for metabolic function. While a slight