Improved intrapulmonary delivery of site-specific PEGylated salmon calcitonin: Optimization by PEG size selection

The purpose of this study was to demonstrate the biological potentials of PEGylated salmon calcitonin (PEG-sCT) derivatives administered intratracheally and their dependences on PEG Mw (1, 2, 5 kDa). Initially, three different PEG-sCT derivatives were site-specifically synthesized by attaching PEG to the Lys¹⁸-amine. In an attempt to examine the pulmonary feasibilities of these derivatives, the following evaluations were undertaken to determine their; (i) proteolytic resistances to pulmonary enzymes, (ii) bioactivities, and (iii) pulmonary pharmacokinetic and pharmacologic profiles. The results obtained showed that the pulmonary stabilities and pharmacokinetic properties of these derivatives were greatly improved by increasing PEG Mw. PEG-sCTs had 10.5-, 40.1-, and 1066.0-fold greater stabilities than that of sCT in rat lung homogenates. Moreover, all pharmacokinetic parameters (AUC_inf, C_max, t_1/2, and others) of these derivatives in endotracheally cannulated rats were significantly improved by PEGylation. Specifically, C_max values increased on increasing PEG Mw, i.e., 78.1 ± 21.1, 102.9 ± 9.1, and 115.2 ± 5.7 for 1, 2, 5 kDa, respectively, vs. 54.8 ± 3.9 ng/mL for sCT. Their circulating t_1/2 values also increased to 53.9 ± 6.0, 100.7 ± 21.7, and 119.4 ± 13.7 min, respectively, vs. 34.6 ± 7.6 min for sCT. Despite having the best properties, Lys¹⁸-PEG_5k-sCT was found to have significantly lower hypocalcemic efficacy than other PEG-sCTs, probably due to its reduced intrinsic bioactivity (∼ 30% vs. sCT). Rather, Lys¹⁸-PEG_2k-sCT showed the most promising pulmonary potential because of its well-preserved bioactivity (> 80% of sCT). Taken together, our findings suggest that the site-specific substitution to peptides like sCT with a PEG of an appropriate size offers optimized therapeutic potential by dual advantages, i.e., (i) increased proteolytic stability and (ii) extended circulating half-life in terms of intrapulmonary delivery.