Pharmacokinetics of midkine with different N-terminal structures in rats

Midkine (MK) is a heparin-binding growth factor that functions in multiple physiological processes, making it a promising drug target for treating various diseases including osteoarthritis (OA). However, the lack of pharmacokinetic studies on MK limits further clinical research. As the N-domain of MK protein appears to be more important for its stability, this study aimed to investigate the pharmacokinetic profiles of recombinant human (rh)MK with different structures at the N-terminus via different administration routes in rats and guinea pigs. A single intramuscular (IM) injection of 1 mg/kg rhMK with or without extended sequences at the N-terminus expressed by E. coli or Pichia was administered to six male SD rats. rhMK concentrations in sequential tail blood samples were measured by ELISA. rhMK without extended N-terminal sequences expressed by Pichia had a greater area under the curve (AUC), slower clearance, and longer half-life in rats following a single IM injection than those of the other rhMK proteins. The AUC values for rhMK after IM and intra-articular (IA) administration were 1523.3 ± 35.2 h × ng/mL and 872.0 ± 36.1 h × ng/mL, whereas the apparent volumes of distribution (Vd/f) were 0.184 ± 0.067 L/kg and 11.6 ± 0.8 L/kg, respectively, suggesting that rhMK was distributed more locally after IA injection than after IM injection as Vd/f magnitude gives a general idea of extent distribution in the body and higher Vd/f represents more locally distribution. rhMK concentration in the articular cartilage was markedly higher than that in serum and reached the highest level at 3 days after a single IA injection in Hartley guinea pigs. As the dose increased from 10 to 50 mg/kg, the AUC increased in a greater-than-dose-proportional manner, suggesting that rhMK exhibits non-linear pharmacokinetics in rats after a single IM injection in this dose range. These results indicated that the N-terminal structure and administration route have substantial effects on the pharmacokinetics of rhMK in rats. Furthermore, rhMK was maintained in articular cartilage with minimal diffusion into the blood following IA injection in Hartley guinea pigs, providing a foundation for clinical research on the use of rhMK for OA treatment via IA delivery.