Improvement of the pharmacological properties of amidated kyotorphin by means of iodination

Abstract

Amidated kyotorphin (L-Tyr-L-Arg-NH₂; KTP-NH₂) shows analgesic properties following systemic administration. Although KTP-NH₂ does not have toxic effects in the liver, its biodistribution is unknown. KTP-NH₂ was radioiodinated to evaluate its biological fate in vivo. Mono-radioiodinated KTP-NH₂ ([¹²⁵I]MIK) was radiochemically stable in vitro, namely in saline at 4 °C up to 1 week, and moderately stable when incubated with human serum at 37 °C. Although the radioiodinated peptide could translocate a cellular model of the blood–brain barrier (BBB), the levels of radioactivity in the brain were minimal, 5 and 10 min after intraperitoneal injection (i.p.). Significant accumulation of ¹²⁵I was found in the thyroid probably reflecting the hydrolysis of the iodine–tyrosine bond by liver deiodinases. HPLC analysis of plasma samples collected 5 min post-injection showed that intact [¹²⁵I]MIK accounted for 72.5% of the total radioactivity, whereas the remaining radioactivity was associated with free radioiodide (I⁻). These findings were subsequently confirmed by the low radioactivity found in the liver and kidney homogenates from rats sacrificed 10 min after i.p. administration. The analgesic activity of mono- and di-iodinated KTP-NH₂ was also evaluated by a hot plate assay showing a delayed peak of maximal efficacy compared to KTP-NH₂ (30 vs. 15 minutes). Overall, the peripheral effects of the peptides cannot be excluded.