Aminoglycoside antibiotics seem to accumulate and persist in the kidney. For a better understanding of this problem, groups of six rats received a single 4 mg/kg i.p. injection of sisomicin and were sacrificed repeatedly from 30 min to 28 days later. Sisomicin concentrations (bioassay) decreased rapidly in the serum, lung and other tissues. There was only a trace at six hours. The situation was totally different for the kidney. Concentrations in the cortex increased up to six hours with a maximum of 99 mug/g, 11 times higher than the peak value in the serum then decreased very slowly to 56, 18, and 7 mug/g, 2, 14 and 28 days, respecitvely, after injection. The concentrations in the medulla were lower than in the cortex but also showed an accumulation and persistence. Similar results were observed with gentamicin. In another experiment, daily injections of sisomicin or gentamicin during seven days demonstrated that the concentrations of both antibiotics six hours after the last injection were nearly three times higher in the cortex and twice as high in the medulla than after a single injection. These data explain why the nephrotoxicity of sisomicin or gentamicin involves chiefly the cortex, increases with the length of the treatment and can persist for several weeks after the last injection. Therapeutic implications need further studies.

No abstract

Little is known about the behaviour of antibiotics in the tissues. To investigate this problem, rats were sacrificed in groups of six, 1, 2, 4, 6, 8, 12 and 18 h after an intraperitoneal injection of 10 mg doxycycline/kg body weight. The antibiotic levels were determined in the nine major organs and in the serum by a microbiological method. One hour after injection, the doxycycline concentrations in all the tissues were already higher than the serum concentrations. After 4 h, the concentration exceeded 2.5 μg/ml in the lungs, muscles, testes and heart, and were much greater in the excretory organs: 11.4 ± 4.1 μg/ml in the liver, 10.2 ± 1.6 in the renal medulla and 27.8 ± 7.0 in the renal cortex. Throughout the experiment, the lung and muscle concentrations were about double the serum concentrations, and this occurred with great regularity. Doxycycline is thus capable of penetrating extremely rapidly and intensely into tissues, while still retaining a great degree of freedom of movement between plasma and tissues.

To study the behavior of antibiotics in the tissues, rats were sacrificed repeatedly in groups of six, after the injection of 25 mg/kg ampicillin, 100 mg/kg cephalothin or 10 mg/kg doxycycline. These antibiotics were bioassayed in ten different organs. Standards were established for each organ by using identical organs, thus avoiding errors caused by tissue binding or inhibition. Penetration into the tissue is very fast. Compared to serum levels, lung, muscle, heart, testicle and spleen, levels are higher for doycycline, lower for ampicillin and variable for cephalothin: for example, lung/serum ratio at 1 h is 2.2, 0.5 and 1.1, respectively; muscle/serum is 2.3, 0.2 and 0.18. The decrease in tissue levels parallels that in the serum for doxycycline, but is slower for ampicillin. The hepatic penetration of cephalothin is less than that of doxycycline or ampicillin. Levels are higher in the renal cortex than in the medulla for doxycycline, lower for cephalothin, and similar for ampicillin. The data enabled calculation of tissular pharmacokinetics. They have practical implications in the selection of antibiotics.

The effect of a subcutaneous injection of an intermediate-acting insulin at bedtime combined with glibenclamide has been evaluated in 16 non-insulin-diabetic patients with secondary failure to respond to oral agents. The patients showed poor metabolic control (HbA1 greater than 11%) after two months on diet and glibenclamide treatment (15 mg.day-1). For 3 months the glibenclamide was continued together with an injection of an intermediate-acting insulin at bedtime in order to maintain fasting blood glucose under 120 mg.dl-1. A significant reduction in fasting blood glucose and HbA1 (15.50 vs 10.35%) and fructosamine (2.03 vs 1.69 mmol.l-1) was observed (230 to 141 mg.dl-1) at a mean insulin dose of 0.28 U.kg-1. The peak blood glucose after a standard test meal was also significantly improved (290 vs 203 mg.dl-1). Two months after the bedtime insulin injection had been withdrawn, only one patient was still being treated with oral agents alone. Except for another patient who dropped out, all the others had to be treated again with insulin because their fasting blood glucose exceeded 180 mg.dl-1. It is concluded that a single subcutaneous injection of an intermediate-acting insulin at bedtime combined with glibenclamide improved fasting and post-meal blood glucose concentrations in non-insulin-dependent patients resistant to diet and oral hypoglycaemic treatment. Almost all of the patients relapsed after insulin was withdrawn.