The Blood Levels and Renal Clearance in Rabbits and Man of an Antibiotic Derived From B. Subtilis (Bacitracin)

Abstract

The renal clearances of penicillins F, G, and X have been shown to approximate the total renal plasma flow (1, 2), varying between 529 and 865 ml. per minute in man, and 23 to 111 ml. per minute in rabbits (2). In consequence of that rapid excretion, the serum concentration of penicillin G decreases after its intramuscular injection in aqueous solution, at an average rate of 2 to 3 per cent of the residual penicillin per minute, and 70 to 80 per cent per hour; and it disappears from the blood even more rapidly after intravenous injection (3,8). A therapeutic agent equal to penicillin in bactericidal activity, but with a slower rate of absorption or excretion, would provide effective levels for longer periods of time, and might be correspondingly more effective than penicillin similarly injected.The antibiotic agent discovered, by Johnson, Anker and Meleney (4) in culture filtrates of a strain of B. subtilis (Tracy), and termed by them bacitracin, possesses some of these properties. As will be here shown, it is excreted by both rabbits and man at a rate which corresponds approximately to the rate of glomerular filtration, rather than to the total renal plasma flow. In consequence, the blood levels observed after its intravenous or intramuscular injection fall off more slowly than do those of penicillin, and a given dosage provides effective levels for longer periods. The implications of these findings with respect to the thera- in facilitating their procurement is gratefully acknowledged. The 3 lots, designated in the tables as A, B-100, and PB-1, had relative gravimetric activities in vitro against the C-203 strain of Streptococcus pyogenes of 100, 165, and 330, respectively. Their activity in terms of the unit as defined by Meleney and his co-workers (4) was 18, 30, and 60 units per mgm., respectively. Lot PB-1 was concentrated in this laboratory from a commercial lot, B-103.Blood specimens in rabbits were obtained by cardiac puncture, and urine specimens by bladder catheterization and irrigation.Method of assay. A modified Rammelkamp-RantzKirby method (5, 6), in which inhibition of hemolysis by the C-203 strain of Streptococcus pyogenes served as the endpoint, uas used to assay the levels of the antibiotic in the blood and urine. The endpoint was not as sharp as with penicillin, and somewhat coarser interpolations were used than had proved feasible with the latter drug (2, 7). The unknown sample, in appropriate dilution as determined by a preliminary spot-assay, was distributed in amounts of 0.8, 0.64, 0.48, 0.4, 0.32, 0.24, 0.2 ... ml., and the volumes brought up to 0.8 ml. with broth. To all the tubes were then added 0.5 ml. of 4 per cent rabbit (or human) blood broth, inoculated with 1/2,500 part of a fresh 6-to 8-hour culture of the organism in blood broth. Difficulties introduced by the occasional contaminated specimen were overcome by removing the assays from the incubator after 6 to 8 hours, when the streptococci had multiplied sufficiently to initiate hemolysis, but before they had been overgrown by th...