Computational models are currently being used by regulatory agencies and within the pharmaceutical industry to predict the mutagenic potential of new chemical entities. These models rely heavily, although not exclusively, on bacterial mutagenicity data of nonpharmaceutical-type molecules as the primary knowledge base. To what extent, if any, this has limited the ability of these programs to predict genotoxicity of pharmaceuticals is not clear. In order to address this question, a panel of 394 marketed pharmaceuticals with Ames Salmonella reversion assay and other genetic toxicology findings was extracted from the 2000-2002 Physicians' Desk Reference and evaluated using MCASE, TOPKAT, and DEREK, the three most commonly used computational databases. These evaluations indicate a generally poor sensitivity of all systems for predicting Ames positivity (43.4-51.9% sensitivity) and even poorer sensitivity in prediction of other genotoxicities (e.g., in vitro cytogenetics positive; 21.3-31.9%). As might be expected, all three programs were more highly predictive for molecules containing carcinogenicity structural alerts (i.e., the so-called Ashby alerts; 61% +/- 14% sensitivity) than for those without such alerts (12% +/- 6% sensitivity). Taking all genotoxicity assay findings into consideration, there were 84 instances in which positive genotoxicity results could not be explained in terms of structural alerts, suggesting the possibility of alternative mechanisms of genotoxicity not relating to covalent drug-DNA interaction. These observations suggest that the current computational systems when applied in a traditional global sense do not provide sufficient predictivity of bacterial mutagenicity (and are even less accurate at predicting genotoxicity in tests other than the Salmonella reversion assay) to be of significant value in routine drug safety applications. This relative inability of all three programs to predict the genotoxicity of drugs not carrying obvious DNA-reactive moieties is discussed with respect to the nature of the drugs whose positive responses were not predicted and to expectations of improving the predictivity of these programs. Limitations are primarily a consequence of incomplete understanding of the fundamental genotoxic mechanisms of nonstructurally alerting drugs rather than inherent deficiencies in the computational programs. Irrespective of their predictive power, however, these programs are valuable repositories of structure-activity relationship mutagenicity data that can be useful in directing chemical synthesis in early drug discovery.

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Human placental lactogen has been found to resemble human pituitary growth hormone very closely in amino acid sequence, about 80% of the residues examined being identical in the two molecules when a revised sequence for growth hormone is used as the basis for comparison. The structural features responsible for the differing biological potency of the two hormones may therefore reside in rather limited regions of primary structure. The observation of internal sequence homologies within the pituitary growth hormone and prolactin and the placental lactogen molecules suggests that these polypeptide hormones may have evolved by genetic reduplication from a smaller common ancestral peptide. This finding directs further attention to subfragments of these molecules as possible possessors of intrinsic somatotrophic and lactogenic activity.Previous studies on pituitary growth hormone and prolactin, and on placental lactogen, have demonstrated a close structural similarity within this group of hormones, and thus provided a basis for their shared biological and immunological properties (1-6). However, complete amino acid sequences have been reported so far only for human growth hormone (1) (growth hormone) and ovine prolactin (3) (prolactin), and detailed intra-species comparisons of structure have not been possible. Recently, we have extended earlier structural studies (2, 5) on human placental lactogen (lactogen) and have determined much of its amino acid sequence. In the course of this work it was noted that discrepancies existed between the amino acid sequence of growth hormone as previously reported by Li and coworkers (1), and that which would have been predicted by homology with the structure of lactogen as determined in our laboratory. This led us to postulate an error in the previous growth hormone sequence, and to undertake a reinvestigation of its primary structure. Our results (7) showed that the previous aminoterminal structure of growth hormone was in fact incorrect, and that a sequence of 15 amino acids containing the single tryptophan of growth hormone, assigned by Li and coworkers to positions 17-31, must reside elsewhere in the molecule, most probably occupying positions 77-91. Our more recent investigations (manuscript in preparation) of the growth-hormone sequence have confirmed that this is indeed the correct location for the tryptophan sequence, and have also demonstrated the presence of the proposed (7) "missing" dipeptide sequence (Leu-Arg) at positions 92 and 93. Work in progress strongly suggests the existence of several further errors in the previous sequence for growth hormone. One of these involves residues 130-132, which we find to be Gly-Ser-Pro rather than Pro-Ser-Gly (see Fig. 4).The present report describes sequence studies on lactogen that demonstrate an extremely close homology with the revised growth hormone structure (7). More surprisingly, we also observed unequivocal internal sequence homology between four different regions of the lactogen structure. Examination of the revised ...

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