Research ArticlesPreparation of a microcrystalline suspension formulation of LysB28ProB29‐human insulin with ultralente properties

The monomeric analogue, LysB28ProB29‐human insulin (LysPro), has been crystallized using similar conditions employed to prepare extended‐acting insulin ultralente formulations. In the presence of zinc ions, sodium acetate and sodium chloride, but without phenolic preservative, LysPro surprisingly forms small rhombohedral crystals with similar morphology to human insulin ultralente crystals with a mean particle size of 20 ± 1 µm. X‐ray powder diffraction studies on the LysPro crystals prior to dilution in ultralente vehicle ([NaCl] = 1.2 M) revealed the presence of T3R3f hexamers. Consistent with human insulin ultralente preparations, LysPro crystals formulated as an ultralente suspension ([NaCl] = 0.12 M) contain T6 hexamers indicating that a conformational change occurs in the hexamer units of the crystals upon dilution of the salt concentration. The pharmacological properties of subcutaneously administered ultralente LysPro (ULP) were compared to ultralente human insulin (UHI) using a conscious dog model (n = 5) with glucose levels clamped at basal. There were no statistically significant differences between the kinetic and dynamic responses of ULP compared to UHI [Cmax (ng/mL): 3.58 ± 0.76, ULP and 3.61 ± 0.66, UHI; Tmax (min): 226 ± 30, ULP and 185 ± 42, UHI; Rmax (mg/kg min): 11.2 ± 1.9, ULP and 13.3 ± 2.0, UHI; and TRmax (min): 336 ± 11, ULP and 285 ± 57, UHI]. Although the Pro to Lys sequence inversion destabilizes insulin self‐assembly and greatly alters the time action of soluble LysPro preparations, this modification has now been found neither to prevent the formation of ultralente crystals in the absence of phenolics nor to compromise the protracted activity of the insulin analogue suspension.