Lack of direct bone effect of tofacitinib, a JAK inhibitor, in juvenile rats and evaluation of the association between offspring growth and femur length.

Bone has recently emerged as a target organ for some Janus kinase (JAK) inhibitors in adult and/or juvenile animal toxicity studies. Oral administration of tofacitinib, a JAK inhibitor, was not associated with clinical or macroscopic effects on bone growth and development in a rat juvenile animal study (JAS) with tofacitinib dosing starting on postnatal day (PND) 21. However, given that previous JAS did not include a targeted evaluation of bone, inclusive of microscopic examination, an additional rat JAS was conducted to further assess this risk. In this subsequent JAS, administration of tofacitinib from PND 7-49 or from PND 21-49 did not result in any direct effects on bone, with no histologic effects on developing bone. The only bone effect in this JAS was nonadverse shorter femur length, which was not considered to be a direct effect of tofacitinib, but rather an indicator of growth delay, as this was associated with lower body weights. There were no effects on femur length or body weight after a 2-month recovery period. To further explore the relationship between body weight and femur length, historical control data were analyzed from control rats in other JAS. This analysis clearly demonstrated that shorter femur length can occur as an indirect effect that is highly associated with lower body weight, consistent with what was observed in the JAS with tofacitinib. These analyses provide a robust and valuable data set to support the interpretation of such data in JAS, and further support the lack of direct effects of tofacitinib on bone growth and development. As with the previously conducted juvenile studies with tofacitinib, the additional JAS did not identify any special JAS-based concerns for use in pediatric patients as young as 2 years of age.

Placental transfer of (125)Iodinated humanized immunoglobulin G2Δa in the Sprague Dawley rat.

Antibody-like biopharmaceuticals cross the placenta by utilizing transport pathways available for transfer of maternal antibodies to the conceptus. To characterize the timing and magnitude of this transfer in the rat, embryo/fetal biodistribution of maternally administered radiolabeled humanized IgG2 was quantified over the course of gestation using gamma counting and whole body autoradiography. The result was humanized IgG2 found in rat embryo/fetal tissues as early as gestation day 11 with a >1000-fold increase in the amount of total IgG2 by day 21. The concentration of IgG2 in rat embryo/fetal tissues generally remained unchanged from gestation day 11 to 17 with a slight increase from day 17 to 21. In addition, fetal-maternal tissue concentration ratios remained stable during organogenesis with a slight increase from gestation day 17 to 21. Based on the empirical amount of antibody present in the embryo/fetus during specific developmental windows, direct antibody binding to biological targets could potentially result in adverse developmental outcomes.