SUBCUTANEOUS INSULIN ABSORPTION EXPLAINED BY INSULINS PHYSICOCHEMICAL PROPERTIES - EVIDENCE FROM ABSORPTION STUDIES OF SOLUBLE HUMAN INSULIN AND INSULIN ANALOGS IN HUMANS

Objective: To study the influence of molecular aggregation on rates of subcutaneous insulin absorption and to attempt to elucidate the mechanism of absorption of conventional soluble human insulin in humans. Research Design and Methods: Seven healthy male volunteers aged 22-43 yr and not receiving any drugs comprised the study. This study consisted of a single-blind randomized comparison of equimolar dosages of I-125-labeled forms of soluble hexameric 2 Zn2+ human insulin and human insulin analogues with differing association states at pharmaceutical concentrations (Asp(B10), dimeric; Asp(B28), mixture of monomers and dimers; Asp89, Glu827, monomeric). After an overnight fast and a basal period of 1 h, 0.6 nmol/kg of either I-125-labeled human soluble insulin (Actrapid HM U-100) or I-125-labeled analogue was injected subcutaneously on 4 separate days 1 wk apart. Absorption was assessed by measurement of residual radioactivity at the injection site by external gamma-counting. Results: The mean +/- SE initial fractional disappearance rates for the four preparations were 20.7 +/- 1.9 (hexameric soluble human insulin), 44.4 +/- 2.5 (dimeric analogue Asp(B10), 50.6 +/- 3.9 (analogue Asp(B28), and 67.4 +/- 7.4%/h (monomeric analogue Asp(B9), Glu(B27). Absorption of the dimeric analogue was significantly faster than that of hexameric human insulin (P < 0.001); absorption of monomeric insulin analogue Asp(B9), Glu(b27) was significantly faster than that of dimeric analogue Asp(B10) (P < 0.01). There was an inverse linear correlation between association state and the initial fractional disappearance rates (r = -0.98, P < 0.02). Analysis of the disappearance data on a log linear scale showed that only the monomeric analogue had a monoexponential course throughout. Two phases in the rates of absorption were identified for the dimer and three for hexameric human insulin. The fractional disappearance rates (%/h) calculated by log linear regression analysis were monomer 73.3 +/- 6.8; dimer 44.4 +/- 2.5 from 0 to 2 h and 68.9 +/- 3.5 from 2.5 h onward; and hexameric insulin 20.7 +/- 1.9 from 0 to 2 h, 45.6 +/- 5.0 from 2.5 to 5 h, and 70.6 +/- 6.3 from 5 h onward. Conclusions: Association state is a major determinant of rates of absorption of insulin and insulin analogues. The lag phase and the subsequent increasing rate of subcutaneous soluble insulin absorption can be explained by the associated state of native insulin in pharmaceutical formulation and its progressive dissociation into smaller units during the absorption process.