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.