Intranasal Delivery of PEGylated Salmon Calcitonins: Hypocalcemic Effects in Rats

To evaluate the hypocalcemic effect of polyethylene gtycol-conjugated salmon calcitonins (PEG-sCT) in rats, mono-PEGylated sCTs (mono-PEG-sCTs) and unmodified sCT were administered via the intranasal route and serum calcium levels were measured by colorimetric assay using o-cresolphthalein. Mono-PEG-sCTs were prepared with different sizes of succinimidyl succinate monomethoxy PEG molecules (PEG_2K, PEG_5K, PEG _12K) and characterized by HPLC and MALDI-TOF mass spectrometry. Nasal instillation of mono-PEG_2K-sCT at a dose of 2 IU/kg resulted in sustained reduction in serum calcium levels over 8 hr, with a maximum reduction (% max_d) of 13% after 6 hr of application. Whereas unmodified sCT showed a transient decrease in serum calcium levels with the maximum reduction (5%) observed after 30 min of administration. The overall reductions in serum calcium levels expressed as the net change in AUC relative to con-trol in 8 hr were 11.9 ± 0.2, 4.6 ± 0.7, and 2.6 ± 0.7% for mono-PEG_2K-, mono-PEG_5K-, and mono-PEG _12K-sCT, respectively, compared to 3.2 ± 0.6% for unmodified sCT. The relative bioavailability of nasally administered 2 IU/kg of mono-PEG_2K-sCT was approximately 4-fold higher than nasally administrated unmodified sCT, and the absolute bioavailability was approximately 91% of intravenously injected sCT in 8 hr. It can be concluded that the intranasal absorption of mono-PEG-sCTs was inversely related to the molecular weights of the PEG attached. Of the PEGylated sCTs examined, mono-PEG_2K-sCT showed the most pronounced hypocalcemic effect. Therefore the intranasal application would probably be an alternative route of administration for mono-PEG-sCTs in achieving sustained calcium-lowering effects.