Based on the following study (and if the math police do
not come after me), if temperature-dependent
degradation is indeed a first-order process, over 98% of the
original insulin activity is present after one year at 37 C
(much longer than one would expect a vial to be used).
Diabetologia 1975 Dec;11(6):581-584
Stability of the 4th International Standard for Insulin.
Storring PL, Greaves PL, Mussett MV, Bangham DR
The stability of the (W.H.O.) 4th International Standard for Insulin,
has been assessed by accelerated thermal degradation studies. This is
a crystalline preparation of insulin, freed from proteolytic enzymes,
sealed in ampoules containing air and with a moisture content of
5--6%. Of the original biological activity 95.8 (92.8--98.9;P = 0.95)%
was retained after storage for 12 years in the dark at 20 degrees C
and 65.7 (63.4--68.1;P = 0.95)% after 14 years at 37 degrees C.
Degradation rate constants were calculated from these data for the
Standard at 20 degrees C and 37 degrees C, assuming first order
kinetics. The degradation constant at 37 degrees C did not differ
significantly from those found in earlier degradation studies at 37
degrees C over shorter periods, thereby supporting the assumption that
the degradation of crystalline insulin, at least at 37 degrees C, is a
first order reaction. Extrapolation of these data suggest that the
Standard stored at -20 degrees C for 20 years would have retained at
least 99.93% (P = 0.95) of its original activity and so for practical
purposes can be considered to be stable.