CEPHEM SULFONES AS INACTIVATORS OF HUMAN-LEUKOCYTE ELASTASE .5. 7-ALPHA-METHOXY-1,1-DIOXOCEPHEM-4-KETONE AND 7-ALPHA-CHLORO-1,1-DIOXOCEPHEM-4-KETONE

Studies on cephem sulfones as inhibitors of human leukocyte elastase ( HLE) have been extended to the new class of cephem 4-ketones. tert-But yl and phenyl ketones were prepared from 4-carboxycephem derivatives, at either the sulfide or sulfone oxidation level, by chemoselective Gr ignard reaction. Obtained products were functionalized with heterocycl othio and acyloxy substituents at C-3', C-2, or both positions. tert-B utyl ketones of the 7 alpha-chlorocephem series were in general at lea st as potent as the corresponding esters at inhibiting the enzyme, but improvements in hydrolytic stability were only marginal. On the other hand, tert-butyl ketones of the 7 alpha-methoxycephem series combined potent biochemical activity with acceptable hydrolytic stability, thu s overstepping the esters, thiolesters, and amides reported previously . In particular, the tert-butyl ketones possessing a heterocyclothio g roup at C-3' or C-2 were at least as active as the corresponding tert- butyl esters but 1 order of magnitude more stable in physiologic buffe rs (pH 7.4, 37 degrees C). Introduction of acyloxy groups at C-2 deliv ered the most potent HLE inhibitors of the cephem class ever reported, with inhibition parameters often outside the determination limits of our standard protocol (second-order rate constant k(on) > 2 000 000 M( -1) s(-1); K-i at steady state <2 nM). Keto-enol tautomerism was found to depress activity and boost hydrolytic stability. Thus, double subs titution with heterocyclic thiols produced compounds with diverging pr operties, according to the extent of enolate formation at the investig ated pH (7.4): the weakly acidic tert-butyl ketones (pK(a) greater tha n or equal to 5.8) proved to be potent inhibitors (k(on) over 10(4) M- l s(-1)) with reasonable hydrolytic stability (t(1/2) = 30-75 h), whil e the phenyl ketones (pK(a) < 4) were fair inhibitors (k(on),, over 10 (3) M(-1) s(-1); K-i at steady state approximate to 50 nM) with hydrol ytic half-lives exceeding 1000 h. Selected compounds efficiently inhib ited the degradation of insoluble bovine neck elastin by HLE in a conc entration-dependent manner. Intracellular HLE of polymorphonuclear leu kocytes was in general unaffected; however, a lipophilic cephem sulfon e apparently able to inactivate the enzyme in living cells was identif ied.