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The expression of target organ toxicity ranges from subtle abnormalities of cellular organelles to permanent loss of organ function. The selective targeting of chemicals for discrete regions and cell types of a given organ is frequently due (besides some pharmacodynamic mechanisms) to the fact that target cells may express unique biochemical or functional characteristics predisposing them to chemically induced injury. In vitro models commonly used in target organ toxicity tests include perfused organ preparations, isolated tissue preparations, single-cell suspensions and tissue culture systems. Although these systems have proved their usefulness for acute toxicity tests, there is still a great need for in vitro models to be used for chronic toxicity tests. Among the systems listed above, the single-cell culture technique may be adapted to long-term study requirements. The example of kidney proximal tubules is taken to illustrate the necessity for extensive characterization of the actual capacities of the models in term of phenotypic profiling, energy status, drug detoxication activities, specific transport systems and organ-specific differentiated functions. LLC-PK1, LLC-RK1, NRK and OK cell lines are compared with primary cultures of rat, rabbit and human proximal tubule cells. The importance of the cell culture environment on the cell phenotypic profile, and its subsequent response pattern to toxicant exposure, are described using gentamicin and platinum derivatives as examples. In terms of experimental strategy, choice of cell type, choice of species of origin, choice of doses, choice of duration, continuous or discontinuous exposure, and whether to study the recovery phase, are crucial issues for designing models mimicking more closely the in vivo situation. The identification of relevant endpoints, allowing discrimination between general cell toxicity and specific organ toxicity, has not been sufficiently explored in vitro. Scientifically based endpoints referring to the background studies conducted by biochemists or physiologists should be selected and included in experimental designs dealing with organ toxicology in vitro. Conceptually, relevant specific target-organ toxicity could be investigated by the use of multiple cell types and by analysis of the difference in concentration between the cytotoxic concentration threshold and the specific endpoint alteration threshold. |
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