A comparison of HPLC and bioassay methods for plasma melanotan‐II (MT‐II) determination: Application to a pharmacokinetic study in rats

The pharmacokinetic profile of the melanotropic peptide, melanotan‐II (MT‐II), was determined in rats following a 0.3 mg kg⁻¹ intravenous dose. Regression analysis of the plasma MT‐II concentrations determined using HPLC and bioassay methods indicated the existence of a significant linear correlation (r = 0.90, p < 0.001). The plasma concentration versus time plots determined using the two assay methods yielded biphasic disposition profiles that were essentially superimposable. The following pharmacokinetic parameters were assessed from plasma concentration versus time data using both methods: C_max, AUC, CL_s, t_1/2β, MRT V_dβ, and V_ss. Statistical comparison showed that the parameters measured by each method were not significantly different (at the 0.05 level) except for t_1/2β, MRT and V_ss. The presence of even one aberrant data point in the β‐phase can significantly influence t_1/2β when only a few data points are available in the β‐phase. Since MRT and V_ss were calculated from t_1/2β it is not surprising that these two parameters also differed between methods.