Dibenzylethylenediamine (DBED) penicillin G labeled with c!4 in the DBED moiety 
or the phenylacetyl moiety was found to remain in the body and at the site of the injection 
for longer than 3 days in rats, while penicillin G remained there for only 8 to 12 hours. 
In one trial more radioactivity from the intramuscular administration was found in the 
intestine than in the urine. Following oral administration 18 to 22 percent of each of the 
penicillins was excreted in the urine within 24 hours. 
No labeled carbon was found in the respiratory CO >. Total urinary excretion of 
biologically active material ranged from 30 to 60 percent of the I.M. dose. Only 2 percent 
of the phenylacetyl groups were split fromthe penicillin molecule and the major degrada- 
tion product was penicilloic acid, a compound obtained in vitro by alkaline cleavage of 
penicillin (29). 
This evidence indicates that penicillin or its metabolites are not stored in the body 
but are excreted, the rate depending on the reactivity or solubility of the antibiotic 
compound. 
Cyclines: Oral administration of 250 mg. of C14 labeled oxytetracycline to humans 
produced little correlation between results of radiochemical and microbiological analysis 
on the same samples of arterial or venous blood, urine, or bile. Radioactivity was found 
in the blood serum and the urine for 7 days but biological activity was found in the serum 
for only 8 hours and in the urine for only 4 days. Evidently both urine and blood serum 
contain cycline metabolites that exert little or no microbiological activity (30). 
Other Antibiotics: The liver in dogs has been shown to forma metabolic product 
from erythromycin that is still active microbiologically which is excreted in the bile but 
not reabsorbed as is the parent compound. 
Carbomycin is apparently almost completely metabolized or inactivated since only 
1 to 2 percent of the dose given by any route of administration can be accounted for (5). 
It has been shown that a biologically inactive compound, isochlortetracycline, is 
formed as a result of the cooking of meat containing chlortetracycline. This compound 
has also been isolated from the G.I. tract after oral administration of chlortetracycline 
(22). 
Present data give some information on the fate and metabolism of penicillin and 
cycline antibiotics but even this is meager and inadequate. The fate and metabolism of 
other antibiotics are as yet unexplored. 
Excretion 
The principal pathways of excretion are the urine and feces which have been dis- 
cussed in all previous sections of the manuscript. The extent of urinary excretion was 
found to vary among individuals but the various antibiotics have general trends in this 
regard. After parenteral administration the recovery rate in the urine averages about 
70 to 80 percent for penicillin, 60 percent for chlortetracycline, about the same for other 
cyclines, 10 to 30 percent for large doses and up to 100 percent for small doses for 
bacitracin, 50 to 90 percent for streptomycin in humans but 100 percent in dogs, 90 
percent for chloramphenicol, 30 to 50 percent for neomycin, and 0.5 to 2 percent for 
erythromycin and carbomycin. After oral administration more oxytetracycline can be 
found in the urine than for chlor- or tetracycline (22 to 40 percent vs. 10 to 20 percent). 
Urinary recovery rates for some of the other antibiotics are: Penicillin 13 to 34 per- 
cent, 5 to 30 percent for chloramphenicol, and zero or very little for bacitracin, strepto- 
mycin, neomycin, and carbomycin. Except for the cyclines bound to bone and the metabolic 
fate of the portions not accounted for are unknown. 
After intravenous administration, a serum half-life of over 2 hours was found for 
novobiocin, chlor- and tetracycline; 1 to 2 hours for streptomycin, erythromycin, and 
oleandomycin; and 0.5 to 1.0 hour for oxytetracycline and penicillin V or G (32). 
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