Development of internal thresholds of toxicological concern (iTTC) for chemicals based on empirical exposures to...

The threshold of toxicological concern (TTC) is an approach to risk assessment that uses acceptable low-level exposure values derived from statistical information for chemicals for which insufficient toxicological information exists. Previously, the TTC for systemic toxicity was derived from the toxicological endpoints of the external dose for each administration route in animal studies. Internal TTC (iTTC), which extends the TTC concept from external to internal dosages, expands the scope of application by standardizing exposure via different routes. Information about blood exposure to pharmaceuticals used in treatment can help determine the chemical levels tolerated in humans. Using the tolerated plasma/serum blood drug concentration of the pharmaceutical as the toxicological point of departure, a new iTTC of 0.6 nM for the no effect concentration were proposed (a conservative risk assessment considers further safety margins). It is based on the 5th percentile values of the normal distribution, which approximates the cumulative empirical distribution, and an uncertainty coefficient to convert the treatment dose to the no-effect dose. Predicting blood concentrations of chemicals to which humans are exposed and comparing them to iTTC with appropriate caution can be used as an approach to risk assessment of systemic toxicity without animal-based testing.