The immune system as a target for antibiotics.

Studies on antibiotics, oxytetracycline (OxyTC) in particular, are presented in this thesis with respect to the influence of these drugs on the immune system of carp and chickens. Special attention was paid to the pharmacokinetic behaviour of OxyTC.<u>Immunology</u><u></u>Carp ( <u>Cyprinus</u><u>carpio</u> ) leukocytes from spleen, pronephros and mesonephros showed a clear <u>in</u><u>vitro</u> mitogenic response. The DNA synthesis of PHA-stimulated pronephric cells was reduced to about 50% of the mitogen-activated controls at therapeutic concentrations of 4 - 6 μg OxyTC/ml. Doxycycline (DC) was even more suppressive: 50% inhibition at 1 - 2.5 μg/ml. The difference between the two tetracycline analogues may relate to the better lipid-solubility of DC. In addition. OxyTC (10 μg/ml) and DC (5 μg/ml) markedly reduced the incorporation of <sup><font size="-1">3</font></SUP>H-thymidine into alloantigen-stimulated pronephric leukocytes by 61% and 98%. respectively. Furthermore. it was demonstrated that OxyTC changed the kinetics of the mitogenic response at relative low concentrations (5 - 15 μg/ml). A dose-dependent delay of 1 - 2 days was observed. In fact. the antibiotic suppression was transient. Obviously. the impairment of cellular functions like DNA synthesis is not due to cell killing.The immuno-modulating effects of OxyTC was also demonstrated for chicken peripheral blood (PBL) and spleen leukocytes. However, the kinetics of the <sup><font size="-1">3</font></SUP>H-thymidine incorporation by lectin-activated PBL showed that the DNA synthesis was severely suppressed in stead of delayed. The production of the amplifying/regulatory factor interleukin 2 (IL-2) was not affected in the presence of the antimicrobial agent. Whereas, the uptake of <sup><font size="-1">3</font></SUP>H-thymidine by IL-2-dependent T cell blasts was strongly reduced. It was concluded that the T cell blast is one of the targets for OxyTC.Investigations in carp on the kinetics of the primary anti-SRBC antibody response, during a short-term OxyTC treatment <u>in</u><u>vivo</u> , revealed that the kinetics of the plaque forming cell response was not significantly affected by OxyTC. In contrast, the number of plaque forming cells was significantly decreased. Furthermore. the anti-SRBC antibody production was delayed by 2 - 4 days. However. within 12 - 14 days post immunization the same antibody levels were detected as in the control group. indicating the temporary effect of OxyTC on the immune response.<u>Pharmacokinetics</u>The objective of the pharmacokinetic studies was to investigate plasma disposition, distribution, bioavailability and elimination of OxyTC in carp, rainbow trout ( <u>Salmo</u><u>gairdneri</u> ) and African catfish ( <u>Clarias</u><u>gariepinus</u> ).Analysis of the plasma drug concentration time curves following intravenous (i.v.) OxyTC administration (60 mg/kg) revealed distinct phases. The two- and three-compartment open model adequately described the plasma drug levels in catfish and trout, respectively. A three-compartment model was also used for carp to derive pharmacokinetic parameters. Compared with mammals, an extended average final elimination half-live was observed in all three fish species following i.v. OxyTC administration: carp: 139.8 h; trout: 89.5 h; catfish: 80.3 h. Between the three species, marked differences were observed with respect to the rate diffusion constants, distribution volume of the central compartment. apparent distribution volume and body clearance.A bioavailability was observed of 80 - 86% following intramuscular (i.m.) injection of OxyTC (60 mg/kg). Whereas, a bioavailability of only 0.6% was achieved in carp with the oral route of administration.<u>Immunology and Pharmacokinetics</u>Oxytetracycline interfered with the immune response in carp. However. data were needed concerning the pharmacokinetic behaviour of this drug in carp. Tissue distribution studies following a single i.m. OxyTC injection (60 mg/kg) revealed that the drug was accumulating in pronephros, bone tissue and scales. After 21 days the average OxyTC concentrations were 2.9. 5.2 and 4.7 μg/ml, respectively. The pronephros is an important lymphoid and haemopoietic organ in fish. Furthermore, the pharmacokinetic data showed that high plasma levels (>50 μg/ml) were detected over a relatively long time period. These data provide more insight into the action of OxyTC upon immuno-modulation.<u>Immuno-pharmacokinetic research</u>When defence mechanisms fail to prevent the establishment of infective micro-organisms in the host. the consequence will be disease. Under these conditions antibiotics are valuable tools for the therapy of bacterial infections. Interference of an antibiotic with the immunological responses will decrease the defensive potential. Therefore, it is important to exclude any immuno- suppression by antimicrobial drugs in animals and man. This is obvious. because the defence mechanisms have to ensure the final elimination of pathogens.In this context pharmacokinetic studies. including tissue distribution, are of great importance. These studies provide data concerning the behaviour of drugs within the body. Extreme differences can be expected with respect to animal species, age and disease status.The combined action of immune system and antimicrobial drugs will increase the defensive capacity. Therefore, it is emphasized that an immuno-pharmacological approach is a prerequisite to prevent immuno- toxicological events and to optimize the use of antibiotics. Consequently, the fundamental aspects of this type of research will support an effective human and veterinary medical management