[Teratological determination of some thalidomide metabolites]

Assessment of the effectiveness of animal developmental toxicity testing for human safety

Evaluations of studies for four well-known human developmental toxicants clearly suggest that a margin of exposure of 1/100th the NOAEL for the most sensitive animal species tested provides adequate safety for the human conceptus. The lowest reported human teratogenic exposures occurred at doses at least one log above the estimated "safe" or acceptable daily exposure based on the most sensitive animal species, that is, 1/100th animal NOAEL. (The MOE ranged from < 1 to 10.). The data and analyses are consistent with the conclusion that, regardless of the type of in utero effect produced in animals, the margin of safety of 100 is likely to protect the human conceptus in utero from developmental perturbation, and it is a scientifically reasonable and conservative default number.

The FDA perspective on the development of stereoisomers

The current regulatory position of the Food and Drug Administration is discussed with regard to the approval of racemates and pure stereoisomers. Circumstances in which stereochemically sensitive analytical methods are necessary to ensure the safety and efficacy of a drug are described. Regulatory guidelines are interpreted for applications for the approval of a pure enantiomer in which the racemate is marketed, for the approval of either a racemate or a pure enantiomer in which neither is marketed, and for clinical investigations to compare the safety and efficacy of a racemate and its enantiomers. Examples of the basis for such regulation are drawn from historical situations (thalidomide, benoxaprofen) as well as currently marketed drugs (arylpropionic acids, disopyramide, indacrinone).

Teratogen metabolism: spontaneous decay products of thalidomide and thalidomide analogues are not bioactivated by liver microsomes

Thalidomide and two analogues, EM87 and EM12, inhibited the attachment of tumor cells to concanavalin A-coated surfaces only if the drugs were first incubated with hepatic microsomes and cofactors. Most agents that inhibit attachment are demonstrated teratogens. Thalidomide undergoes spontaneous hydrolysis to at least 12 products in saline buffered to a pH of greater than 7. These hydrolysis products did not inhibit attachment nor could they be activated to inhibitory products with hepatic microsomes. Similarly EM12 and EM 87 hydrolysis products were neither inhibitory nor substrates for activation. If the three drugs were incubated in buffered saline, there was a progressive decline in their ability to act as substrates for activation to an inhibitory product. It was possible to remove microsomes from the incubation mixture following drug activation by centrifugation. This microsome-free mixture inhibited cell attachment. When mouse ovarian tumor (MOT) cells were added to the microsome-free mixture, attachment was inhibited. However, if the activated drugs were incubated in saline, there was a progressive decline in their ability to inhibit attachment. Decay rates differed for the three compounds. At a pH of 7.4, thalidomide, EM87, and EM12 required 3 h, 1h and 6h, respectively, to decay to control levels. These relative rates of decay are consistent with the relative teratogenicity of the three drugs.

Teratogen metabolism: activation of thalidomide and thalidomide analogues to products that inhibit the attachment of cells to concanavalin A coated plastic surfaces

Thalidomide metabolites inhibited the attachment of tumor cells to concanavalin A coated polyethylene surfaces. Thalidomide, itself, was non-inhibitory. Thalidomide activation to inhibitory products required hepatic microsomes, an NADPH-generating system, and molecular oxygen. Production of inhibitory metabolites was unaffected by either epoxide hydrolase or 1,2-epoxy-3,3,3-trichloropropane (TCPO), an inhibitor of epoxide hydrolase endogenous to hepatic S9 fraction. Therefore, the attachment inhibitor was probably not an arene oxide. Inhibition was not accompanied by cytotoxicity, as judged by trypan blue exclusion. Although uninduced hepatic microsomes from mice, rats and dogs had similar abilities to activate thalidomide, microsomes from Aroclor 1254 induced rats were relatively inactive in the system. Inhibitory metabolites were generated from the thalidomide analogues EM8 , EM12 , EM16 , EM87 , EM136 , EM255 , E350 , phthalimide, phthalimido-phthalimide, indan, 1- indanone and 1,3- indandione . Glutarimide , glutamic acid and phthalic acid did not activate to inhibitory products.

[Embryotoxicity and teratogenicity of derivatives of 1,3-indandion (author's transl)]

The embryotoxicity and teratogenicity of 1,3-indandion and of 3 of its derivatives were proved. Pregnant SWS mice were given single i.p. injections on day 9 of gestation. A mixture of physiologic saline and TWEEN20 (3:1) was used as a solvent. The results refute the hypothesis of the action of thalidomide as an acylating agent responsible for the embryotoxic and teratogenic effect. The results favor instead the hypothesis of an intercalation of the flat purine-like molecule moieties of thalidomide and some of its analogues between base pairs of the DNA double helix.