Methylene Blue Trihydrate is a dye which is prescribed in human and veterinary medicine to treat chemically- induced methemoglobinemia. MBLU is also used in obstetrics to determine the presence of multiple amnionic sacs or premature rupture of the membranes. Clinical reports suggest MBLU may be fetotoxic and studies in pregnant rats indicate MBLU may have an adverse impact on embryo/fetal development. In the current study, the effects of MBLU on maternal health and embryo/fetal growth, viability, and morphological development were determined in pregnant New Zealand white rabbits. MBLU (0, 50, 100, or 150 mg/kg/day) was administered by gavage during the major period of organogenesis (gd 6 -19). Maternal clinical signs, body weight, and food consumption were monitored at regular intervals from gd 0 to 30. On gd 30, fetuses were removed from the does and examined for evidence of MBLU-induced developmental toxicity.
At 100 mg/kg/day MBLU there was one maternal death which may have been treatment-related; however, no maternal deaths were observed at 150 mg/kg/day. MBLU did not adversely affect maternal food consumption or body weight. However, maternal body weight gain was significantly decreased during early treatment (gd 6 to 9) in the 100 and 150 mg/kg/day groups. Maternal body weight change during the entire treatment period (gd 6 to 19) tended to be lower in the MBLU-treated groups (significant trend, only). Maternal liver and spleen weights were not affected by MBLU administration.
Examination of the uterine contents on gd 30 failed to provide definitive evidence for MBLU-induced developmental toxicity. Although there were significant increasing trends for several developmental endpoints, no group-wise significant effects of MBLU were detected for any indicator of embryo/fetal growth, viability, or morphological development (see Table 1). The incidence of umbilical hernia appeared to be increased at the high dose, relative to concurrent and historical controls, although this increase did not reach statistical significance for this study.
These results suggest that the high dose (150 mg/kg/day) of MBLU approaches the lowest observed adverse effect level because of the presence of significant dose-related trends for several developmental endpoints. This interpretation is supported by an increase in abortions (7 of 9 does) at 200 mg/kg/day in the range-finding study, as well as an increase in the number of resorptions per litter in the 2 remaining does in that group (NTP, 1992). The LOAEL for maternal toxicity in this study was 100 mg/kg/day based upon transient reductions in maternal weight gain during the first three days of treatment and one treatment-related death at that dose; the maternal no observed adverse effect level was 50 mg/kg/day.