J. Med. Chem. 1995,38, 220-222
220
Aspartyl u-((Diphenylphosphiny1)oxy)methyl Ketones as Novel Inhibitors of Interleukin-18 Converting Enzyme. Utility of the Diphenylphosphinic Acid Leaving Group for the Inhibition of Cysteine Proteases Roland E. Dolle,*J Jasbir Singh,? David Whipple,? I. Kelly OsifoJ Gary Speier,? Todd L. Graybill,? Jill S. Gregory,$Alex L. Harris,* Carla T. Helaszek,f Robert E. Miller,$ and Mark A. Ator* Departments of Medicinal Chemistry and Biochemistry, Sterling Winthrop Pharmaceuticals Research Division, 1250 South Collegeville Road, P.O. Box 5000, Collegeville, Pennsylvania 19426 Received October 19, 1994
One of the most significant advances in the design of cysteine protease inhibitors was the discovery of the peptidyl a-(aryloxy)- (i) and a-((arylacy1)oxy)methyl ketone (ii) classes of inhibit0r.l In their seminal publication, Krantz and Smith demonstrated that these agents are potent and selective irreversible inactivators of this protease family.lb They showed that unlike the corresponding peptide chloromethyl ketones, i and ii possess low chemical reactivity and are in essence mechanism-based in their covalent labeling of the active site cysteine.l Dipeptides bearing an (arylacy1)oxy (benzoate) leaving group are orally bioavailable and reportedly lack gross toxicological liabilities.2 In conjunction with our research interest regarding the discovery of novel cysteine protease inhibitors, particularly for interleukin-l/3 converting enzyme (ICE),3v4p5we have sought to identify new classes of quiescent affinity labelse6 Our recent report concerning the inhibition of ICE by aspartylmethyl ketones iii containing the ((1phenyl-3-(trifluoromethyl)pyrazol-5-yl)oxy) (or F"P)moiety is an example of such a new class of agent.4 In this communication, we report on aspartyl a-((diphenylphosphiny1)oxy)methylketones iv as time-dependent inhibitors of ICE. We also demonstrate the utility of the diphenylphosphinate leaving group in inhibiting the cysteine proteases, cathepsin B and calpain I.
I:n=O li: n = 1
ill
u
Several years ago we initiated a search for functionality other than phenols and benzoates to serve as leaving groups in the peptidyl-COCHS type of inhibitor. This search for novel leaving groups was prompted by the t
Department of Medicinal Chemistly.
* Department of Biochemistry.
report of Krantz and Smith who observed the rate of cathepsin B inactivation by i and ii is strongly dependent on the benzoate or phenol leaving group PKa.lb However, the potency-pKa relationship is not ab~olute.~ This suggested to us that perhaps the inherent structure of the leaving group may be important for enzyme affinity. Tetrazoles, tetronic acids, heterocyclic acids, phosphorous-based acids, and many other structural classes were examined as potential leaving groups. An important selection criteria for leaving group screening was that the functionality react cleanly in an sN2 fashion with peptide bromomethyl ketones in DMF using NaH or KF as the base. Three peptide scaffolds, Z-Asp, Z-Phe-Ala, and Z-Leu-Phe, were routinely employed in our screening strategy. The selection of the scaffolds was based on their known specificity for ICE,5 cathepsin B,la and calpain I.8 This exercise led to the identification of peptidyl a-((diphenylphosphiny1)oxy)methyl ketones as a new, generic class of cysteine protease inhibitor. Scheme 1highlights the synthesis of the novel diphenylphosphinate inhibitors 1-5 and related analogs 6-9.9 The bromomethyl ketones 10-12 were prepared from the corresponding peptide acids via decomposition of the intermediate diazo ketones with HBr-HOAc.lb15 The reaction of 10- 12 with diphenylphosphinic acid (Aldrich) proceeded smoothly (1.1 equiv of diphenylphosphinic acid, 1.0 equiv of bromomethyl ketone, 1.5 equiv of KF, DMF, 25 "C, 2 h) to give the ((diphenylphosphiny1)oxy)methylketones 4,5,and 13. Treatment of tertbutyl ester 13 with trifluoroacetic acid (TFA) in CH2Clz (0.5 M solution of 13 in 25% v/v TFA-CHzClz, 25 "C, 3 h) furnished the aspartyl methyl ketone 1. As demonstrated previously for the AspCHzPTP series: the benzyloxycarbonyl protecting group in ester 13 was readily removed by catalytic hydrogenation (ambient Hz pressure and temperature) in absolute ethanol (0.01 M) containing 4 equiv of 6 M aqueous HC1. Under these reaction conditions, amine hydrochloride 14 was obtained in virtually quantitative yield. Amine 14 was coupled (using BOP-HOBT or TPTU-HOBT as the coupling reagents)1° to commercially available Z-ValOH and Z-Val-Ala-OH, affording the desired di- and tripeptide methyl ketones 2 and 3 following TFA deprotection. Inhibitors 6-9 were prepared from Z-Val-AspCHzBr and Z-Val-Ala-Asp-CHzBrand phenylphosphinic acid (Aldrich), dimethylphosphinic acid (Aldrich), and bis(4-chloropheny1)phosphinic acid,ll using reaction conditions analogous to those used in the preparation of 1. The mono-, di-, and tripeptide aspartyl-based (phosphiny1oxy)methyl ketones (AspCH2DPP) 1-3 display potent time-dependent inhibition of ICE with second order rate constants of 11800 M-l s-l, 50 200 M-l s-l and 117 000 M-l s-l (Table 1). The trend toward faster rates of inactivation upon increasing the peptide inhibitor length is consistent with that observed for the Asp((2,6-dichlorobenzoyl)oxy)methylketone (AspCHzDCBI4 and Asp-((1-phenyl-3-(trifluoromethyl)pyrazol-5-yl)oxy~methyl ketone ( A ~ ~ C H Z P Tclasses P ) ~ of ICE inhibitor. The phenyl rings of the (diphenylphosphiny1)oxy departing group are important for potency since inhibitors 6 and 7 show attenuated activity relative to 2. In inhibitor 6 a phenyl moiety is replaced with a hydrogen atom, while in inhibitor 7 both the aryl rings are 0 1995 American Chemical Society
Communications to the Editor
Journal of Medicinal Chemistry, 1995, Vol. 38,No. 2 221
Scheme 1. Synthesis of the (Phosphiny1oxy)methylKetones 1-9 (Absolute Stereochemistry is as Shown) HOP(0)Phz
KF
R 2 - H N G B r
Z = benzyloxycarbonyl
R2-HN
0 10: R1 = CHpCOptBu; R2 5 2 11:R1=Me;R2=2-Phe 12: R1 = CH2Ph; R2 = 2-Leu
13
H2-PWC
TFAc
13: R1 = CHZCOztBU; Rz = 2 1: R1 = CHzC02H; R2 2 4 R, = Me; R2 = Z-Phe 5: R1 CH2Ph; R2 = Z-LW
1) Z-Val-OH or 2-Val- Ala-OH; BOP-HOBT, Rz-HN 2) TFA
HCI'HzN
___)
EtOH-HCI
2 R = 2-Val 3: R = 2-Val-Ala
14
7
6
CI 0: R = 2-Val 9: R = 2-Val-Ala
Table 1. Evaluation of Inhibitors 1-9 and Related Analogs 15-20 against ICE. Cathepsin B, and Caloain I ~~
~
KObd[I1(M-ls-l) inhibitor Z-Asp-CHzDPPd (1) Z-V~-A~D-CH~D (2) PP Z-Val-Ala-Asp:CHzDPP (3) Z-Val-Asp-CHzHPPd (6) Z-Val-ASP-CHzDMPd (7) Z-Val-Asp-CHDCPPd (8) Z-Val-Ala-Asp-CHzDCPP(9) Z-Phe-Ala-CHzDPP (4) Z-Leu-Phe-CHzDPP (5) Z-Phe-Ala-CHzDCBd(15) Z-Leu-Phe-CHzDCB (16) Z-Val-Asp-CH2DCBe(19) Z-Phe-Ala-CHzPTP" (17) Z-Leu-Phe-CHzPTP (18) Z-Val-Asp-CHzPTPf(20)
ICEnJ cathemin BbJ calnain IcJ 11800 50200 117000
