The absolute bioavailability of rabbit muscle creatine phosphokinase after intramuscular administration.

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Abstract
A 2-period crossover study was carried out to determine the rate and the extent of bioavailability of rabbit muscle creatine phosphokinase (CPK) after i.m. administration to 6 male rabbits. The plasma profile of CPK after i.m. administration (500 U/kg body wt in 0.8 ml of 2% rabbit serum albumin solution into the vastus lateralis muscle) showed a plateau around 4-12 h. I.v. administration of CPK was performed at doses of 300 and 500 U/kg body wt. The mean clearance/body wt and Vdss/body wt were 0.00604 .+-. 0.00094 l/h/kg (mean .+-. SD., n = 6) and 5.32 .+-. 1.47%, respectively. The mean residence time (MRTim) and mean absorption time (MATim) of CPK after i.m. administration were 20.7 .+-. 4.2 and 11.9 .+-. 5.1 h, respectively. Since MATim was long relative to MRTim, the rate of bioavailability was slow and the residence time distribution appeared to be skewed. The value of MATim/MRTim, reflecting the contribution of moment of input function to the plasma activity of CPK, was 55.4 .+-. 11.6%. The variance of residence time (VRTim) and variance of absorption time (VATim) were 608 .+-. 107 and 505 .+-. 122 h2. The absorption and disposition of CPK after i.m. administration were rather complicated. The mean fraction of CPK absorbed was 0.311 .+-. 0.149. Presumably, the incomplete access of CPK to the systemic circulation was due to extravascular inactivation.