Stable insulin for implantable delivery systems: in vitro studies with different containers and solvents

The stability of a new insulin formulation (lyophilized U100 insulin, Organon) was investigated in vitro in conditions reproducing those of in vivo implanted devices, i.e., constant horizontal agitation at 37 degrees C for 4 wk in various containers and 8 wk in different solvents. Physical stability was assessed by ultraviolet absorption, chemical stability by HPLC, and biological stability by hypoglycemia tests in mice. Insulin precipitated in glass vials but remained clear and active in polyethylene reservoirs and after passage through catheter and pumps in motion, although only 83-90% of insulin was delivered chemically intact. In acidic solvent, insulin showed a major gradual transformation into deamidized derivatives (up to 78% after 8 wk), although still fully active and clear, as expected from previously published excellent in vivo results with acidic insulins. Heparin addition to neutral insulin solution (500 IU/ml) did not alter the properties of the two compounds and might thus be tried to prevent in vivo catheter obstruction due to fibrin deposition.