Establishment of a threshold of toxicological concern for pharmaceutical intermediates based on historical repeat-dose data and its application in setting health based exposure limits

Availability of toxicological data for pharmaceutical intermediates (IMs) used in the manufacture of small molecules is often limited. Scarcity of data - in particular, repeat-dose toxicity (RDT) - renders the calculation of health-based exposure limits (HBELs) problematic. Establishment of HBELs, including occupational exposure limits (OELs) and permitted daily exposures (PDEs) facilitating worker and patient safety respectively, is however essential. Historic 28-day oral rodent toxicity data was analysed for 103 GSK isolated IMs. No-observed (adverse) effect levels (NO(A)ELs) and critical effects were extracted. The 5th percentile (p05) of the NO(A)EL distribution was 15 mg/kg/day. Substance specific HBELs were calculated, selecting the NO(A)EL as the Point of Departure (PoD); 99% of IMs (n = 102) were assigned an oral PDE ≥1000 μg/day and OEL ≥100 μg/m³. A default oral PDE of 1000 μg/day and OEL of 100 μg/m³ is thus proposed for IMs. Evaluation of an additional PoD - benchmark dose lower confidence limit (BMDL) - further supported the default HBELs. The default oral PDE can also serve as a threshold of toxicological concern (TTC) for IMs. Default limits can aid in setting HBELs for novel data-poor IMs, as well as supporting waiving of RDT in the future through read-across.