Potent dipeptidylketone inhibitors of the cysteine protease cathepsin K.

Cathepsin K (EC 3.4.22.38) is a cysteine protease of the papain superfamily which is selectively expressed within the osteoclast. Several lines of evidence have pointed to the fact that this protease may play an important role in the degradation of the bone matrix. Potent and selective inhibitors of cathepsin K could be important therapeutic agents for the control of excessive bone resorption. Recently a series of peptide aldehydes have been shown to be potent inhibitors of cathepsin K. In an effort to design more selective and metabolically stable inhibitors of cathepsin K, a series of electronically attenuated alkoxymethylketones and thiomethylketones inhibitors have been synthesized. The X-ray co-crystal structure of one of these analogues in complex with cathepsin K shows the inhibitor binding in the primed side of the enzyme active site with a covalent interaction between the active site cysteine 25 and the carbonyl carbon of the inhibitor.

Use of papain as a model for the structure-based design of cathepsin K inhibitors: crystal structures of two papain-inhibitor complexes demonstrate binding to S'-subsites.

Papain has been used as a surrogate enzyme in a drug design effort to obtain potent and selective inhibitors of cathepsin K, a new member of the papain superfamily of cysteine proteases that is selectively and highly expressed in osteoclasts and is implicated in bone resorption. Here we report the crystal structures of two papain-inhibitor complexes and the rational design of novel cathepsin K inhibitors. Unlike previously known crystal structures of papain-inhibitor complexes, our papain structures show ligand binding extending deep within the S'-subsites. The two inhibitor complexes, carbobenzyloxyleucinyl-leucinyl-leucinal and carbobenzyloxy-L-leucinyl-L-leucinyl methoxymethyl ketone, were refined to 2.2- and 2.5-A resolution with R-factors of 0.190 and 0. 217, respectively. The S'-subsite interactions with the inhibitors are dominated by an aromatic-aromatic stacking and an oxygen-aromatic ring edge interaction. The knowledge of S'-subsite interactions led to a design strategy for an inhibitor spanning both subsites and yielded a novel, symmetric inhibitor selective for cathepsin K. Simultaneous exploitation of both S- and S'-sites provides a general strategy for the design of cysteine protease inhibitors having high specificity to their target enzymes.

BCL-2 but not its Epstein-Barr virus-encoded homologue, BHRF1, is commonly expressed in posttransplantation lymphoproliferative disorders.

Posttransplantation lymphoproliferative disease (PTLD) is virtually always associated with Epstein-Barr virus (EBV) infection. BCL-2 and other proteins that confer resistance to apoptosis have been implicated in the pathogenesis of a variety of malignancies including lymphomas. One EBV protein, BHRF1, is a homologue of BCL-2, whereas another, the latency membrane protein 1 (LMP-1), upregulates BCL-2 expression in vitro. In the present study, we used immunohistochemistry to study the expression of these viral and cellular proteins as well as a variety of other EBV-encoded proteins in PTLD. BHRF1 was not detected in any PTLD specimen, whereas BCL-2 was shown in 12 of 17 lesions examined. With one exception, all LMP1-positive cases also expressed BCL-2 and the absence of LMP1 was always associated with a lack of BCL-2 expression. The results do not support a role for the EBV homologue of BCL-2 in PTLD, but they do support a role for viral induction of BCL-2 expression.

Aggressive treatment for postcardiac transplant lymphoproliferation.

Posttransplant lymphoproliferative disorder (PTLD) is a frequently fatal complication of organ transplantation, occurring in 2% to 6% of cardiac recipients. Treatment remains poorly defined. Reduction in immunosuppression is effective in a proportion of cases, but mortality on the order of 80% is reported for patients requiring chemotherapy. The reason for such poor outcomes is unclear, but may be partly caused by the concomitant use of immunosuppressives. Nineteen consecutive cardiac recipients with PTLD were studied retrospectively in terms of clinical features and outcome. Patients were managed according to a uniform treatment approach. Initial therapy was a trial of reduced immunosuppression with concomitant acyclovir followed, if unsuccessful, by aggressive combination chemotherapy. The regimen used was predominantly ProMACE-CytaBOM. Six patients with phenotypically polyclonal PTLD presented less than 6 months after transplantation (median 6 weeks). Only 1 of 4 (25%) treated patients responded to reduced immunosuppression; the remainder died of multiorgan failure. Thirteen patients presented with phenotypically monoclonal disease > or = 6 months after transplantation. In 8 of 12 (75%) treated patients initial therapy was reduction in immunosuppression. None achieved complete remission (CR) and 2 experienced fatal rejection. Two patients achieved durable surgical CR. The remaining 8 patients received chemotherapy; 2 of 8 (25%) died during treatment, 6 of 8 (75%) achieved CR. None have relapsed, at a median duration of follow-up of 38 months. Neutropenic sepsis and subclinical doxorubicin cardiotoxicity at a mean cumulative dose of 63 mg/m2 were the principal toxicities. Our data indicate that aggressive chemotherapy is both feasible and effective in phenotypically monoclonal PTLD refractory to reduced immunosuppression. ProMACE-CytaBOM is well suited to cardiac recipients, minimizing doxorubicin exposure and obviating the need for concurrent immunosuppressives.

Inhibition of HIV-1 protease in infected T-lymphocytes by synthetic peptide analogues.

The gag and pol genes of the human immunodeficiency virus type 1 (HIV-1) (ref. 1) are translated as two polyproteins, Pr55gag and Pr160gag-pol (refs 2-6), which are subsequently cleaved by the action of a virus-encoded protease into the four structural gag proteins of the virion core (p17, p24, p7 and p6) and the pol-encoded enzymes essential for retrovirus replication (protease, reverse transcriptase, ribonuclease H, and endonuclease). Mutational inactivation of the proteases of HIV-1 and other retroviruses results in immature, non-infectious virions, indicating that exogenous inhibition of the protease may represent an attractive approach to anti-AIDS therapy. Here we demonstrate that synthetic peptide analogues, which are potent inhibitors of purified HIV-1 protease, inhibit the processing of the viral polyproteins in cultures of HIV-1-infected T lymphocytes and attenuate viral infectivity.

Magnetic resonance imaging: a review of basic principles and potential use in otolaryngology.

Magnetic Resonance imaging (MRi) is a new imaging technique. The authors review the basic principles and present several cases comparing MRi with CT scanning and surgical findings. The major advantages of MRi include lack of ionizing radiation, non-invasiveness and absence of interpetrous artifact when visualizing the posterior fossa. Magnetic Resonance imaging of the posterior fossa, especially of the brain stem and cranial junction, has significant advantages over imaging by CT scanning.