Preclinical Development of EBI-005: An IL-1 Receptor-1 Inhibitor for the Topical Ocular Treatment of Ocular Surface Inflammatory Diseases.

OBJECTIVE: Topical interleukin (IL)-1 receptor (R)1 blockade is therapeutically active in reducing signs and symptoms of dry eye disease. Herein, we describe in vitro and in vivo nonclinical Investigational New Drug (IND)-enabling studies of EBI-005, a novel protein chimera of IL-1ß and IL-1 receptor antagonist (IL-1Ra or anakinra) that potently binds IL-1R1 and blocks signaling. These studies provide an assessment of receptor affinity, drug bioavailability, immunogenic response, safety, and tolerability in mice and rabbits. METHODS: In vitro and in silico along with Good Laboratory Practices (GLP) and non-GLP in vivo studies in mice and rabbits assessed the topical ocular and systemic immunogenicity and toxicology of EBI-005. Animals were treated with EBI-005 once daily subcutaneously or four times daily by topical ocular administration for up to 6 weeks (with 2-week recovery phase). RESULTS: EBI-005 has 500 times higher affinity than anakinra to IL-1R1. Predictive immunogenicity testing suggested that EBI-005 is not more immunogenic. Systemic bioavailability of EBI-005 is low (1.4% in mice and 0.2% in rabbits) after topical ocular administration. EBI-005 penetrated into the anterior ocular tissues within 15 min of topical ocular administration. However, it is low or undetectable after 4 hr and does not form a depot after repeated topical ocular administration. EBI-005 was safe and well tolerated, and exposure to drug was maintained despite an antidrug antibody response after systemic administration, based on IND-enabling toxicology and safety pharmacology studies. CONCLUSIONS: Ocular doses of EBI-005 at 50 mg/mL in mice and rabbits totaling 0.15 mg/eye in mice and 1.5 mg/eye in rabbits, administered 4 times daily, did not produce adverse effects, and demonstrated excellent bioavailability in target tissues with low systemic exposure. In addition, immunogenic response to the drug did not cause adverse effects or diminish the drug's activity in most cases. The results support drug administration of the highest anticipated human clinical study dose of a 20 mg/mL solution (40 µL 3 times daily in each eye).

Multicenter Study of a Novel Topical Interleukin-1 Receptor Inhibitor, Isunakinra, in Subjects With Moderate to Severe Dry Eye Disease.

OBJECTIVES: Isunakinra, formerly known as EBI-005, is a novel interleukin (IL)-1 receptor inhibitor developed for topical treatment of patients with dry eye disease (DED). This phase 1b/2a multicenter, double-masked, randomized, vehicle controlled environmental trial assessed the safety and biological activity of isunakinra in patients with moderate to severe DED. METHODS: Subjects (N=74) were randomized to vehicle (placebo) or isunakinra (5 or 20 mg/mL) 3×/daily for 6 weeks. Evaluations included safety, tolerability, biological activity for signs (corneal fluorescein staining [CFS]), symptoms (pain or sore eyes and total Ocular Surface Disease Index [OSDI]), and reduction in rescue artificial tear use. RESULTS: Topical administration of isunakinra (5 and 20 mg/mL) was safe and well tolerated and resulted in clinically relevant improvements in symptoms (OSDI score, painful/sore eye component of OSDI) and signs (total CFS) compared with baseline with no dose response. OSDI scores improved from baseline by 38% (18.9 points) at 6 weeks and CFS scores improved by 33% (3 points) in the isunakinra groups. These changes were not statistically significant compared with the vehicle. Use of artificial rescue tears was significantly reduced in the isunakinra treatment groups (mean=9 vials) compared with vehicle (mean=31 vials). The differences between isunakinra and vehicle treatments were more pronounced in subjects with OSDI scores less than 50 at baseline. CONCLUSIONS: Isunakinra was safe, well tolerated and showed clinically meaningful improvements in signs and symptoms of DED. These results encouraged the design of an adequately powered study to characterize the safety and efficacy of isunakinra in ocular surface diseases.

A Phase 2 Exploratory Study of a Novel Interleukin-1 Receptor Inhibitor (EBI-005) in the Treatment of Moderate-to-Severe Allergic Conjunctivitis.

OBJECTIVES: Many allergic conjunctivitis (AC) patients are inadequately treated with conventional therapies or require steroids. EBI-005 was developed to address the late phase allergic response. This study's objectives were to evaluate two adapted clinical models for this indication and to assess safety and biological activity of EBI-005 in AC. METHODS: In this randomized, double-masked, vehicle-controlled study, 159 subjects with moderate-to-severe AC were randomized to topical EBI-005 (5 mg/mL) or vehicle control given 3 times per day and repeatedly challenged with allergen using an adaptation of 2 clinical models of AC. Subjects were assigned to repetitive aerosolized challenge in an allergy chamber (Environmental Exposure Chamber, EEC), or repetitive challenges with a direct conjunctival allergen challenge (Conjunctival Allergen Provocation Test, CAPT). RESULTS: In the EEC, the prespecified primary endpoint of ocular itching was not met. In the CAPT, EBI-005-treated subjects showed clinically meaningful, statistically significant improvements in ocular itching compared with vehicle control at the final 2 efficacy time points, visit 6 (P=0.033) and visit 7 (P=0.046). EBI-005-treated subjects showed statistically significant improvement compared with vehicle control for ocular tearing (P=0.027 and P=0.044) and nasal symptoms (P=0.004 and P=0.011) at visit 6 and visit 7. EBI-005 was well tolerated. CONCLUSIONS: These results support use of an adapted, multiple-challenge, direct conjunctival allergen model to assess efficacy of EBI-005 in late phase AC. In the CAPT, EBI-005 showed statistically significant improvements in clinically meaningful symptoms (ocular itching, tearing, and nasal symptoms) at multiple time points for moderate-to-severe AC subjects.

Crystal structure of an HSA/FcRn complex reveals recycling by competitive mimicry of HSA ligands at a pH-dependent hydrophobic interface.

The long circulating half-life of serum albumin, the most abundant protein in mammalian plasma, derives from pH-dependent endosomal salvage from degradation, mediated by the neonatal Fc receptor (FcRn). Using yeast display, we identified human serum albumin (HSA) variants with increased affinity for human FcRn at endosomal pH, enabling us to solve the crystal structure of a variant HSA/FcRn complex. We find an extensive, primarily hydrophobic interface stabilized by hydrogen-bonding networks involving protonated histidines internal to each protein. The interface features two key FcRn tryptophan side chains inserting into deep hydrophobic pockets on HSA that overlap albumin ligand binding sites. We find that fatty acids (FAs) compete with FcRn, revealing a clash between ligand binding and recycling, and that our high-affinity HSA variants have significantly increased circulating half-lives in mice and monkeys. These observations open the way for the creation of biotherapeutics with significantly improved pharmacokinetics.

Design of a superior cytokine antagonist for topical ophthalmic use.

IL-1 is a key inflammatory and immune mediator in many diseases, including dry-eye disease, and its inhibition is clinically efficacious in rheumatoid arthritis and cryopyrin-associated periodic syndromes. To treat ocular surface disease with a topical biotherapeutic, the uniqueness of the site necessitates consideration of the agent's size, target location, binding kinetics, and thermal stability. Here we chimerized two IL-1 receptor ligands, IL-1ß and IL-1Ra, to create an optimized receptor antagonist, EBI-005, for topical ocular administration. EBI-005 binds its target, IL-1R1, 85-fold more tightly than IL-1Ra, and this increase translates to an ∼100-fold increase in potency in vivo. EBI-005 preserves the affinity bias of IL-1Ra for IL-1R1 over the decoy receptor (IL-1R2), and, surprisingly, is also more thermally stable than either parental molecule. This rationally designed antagonist represents a unique approach to therapeutic design that can potentially be exploited for other ß-trefoil family proteins in the IL-1 and FGF families.

CT322, a VEGFR-2 antagonist, demonstrates anti-glioma efficacy in orthotopic brain tumor model as a single agent or in combination with temozolomide and radiation therapy.

Glioblastomas are among the most aggressive human cancers, and prognosis remains poor despite presently available therapies. Angiogenesis is a hallmark of glioblastoma, and the resultant vascularity is associated with poor prognosis. The proteins that mediate angiogenesis, including vascular endothelial growth factor (VEGF) signaling proteins, have emerged as attractive targets for therapeutic development. Since VEGF receptor-2 (VEGFR-2) is thought to be the primary receptor mediating angiogenesis, direct inhibition of this receptor may produce an ideal therapeutic effect. In this context, we tested the therapeutic effect of CT322, a selective inhibitor of VEGFR-2. Using an intracranial murine xenograft model (U87-EGFRvIII-luciferase), we demonstrate that CT322 inhibited glioblastoma growth in vivo and prolonged survival. Of note, the anti-neoplastic effect of CT322 is augmented by the incorporation of temozolomide or temozolomide with radiation therapy. Immunohistochemical analysis of CT322 treated tumors revealed decreased CD31 staining, suggesting that the tumoricidal effect is mediated by inhibition of angiogenesis. These pre-clinical results provide the foundation to further understand long term response and tumor escape mechanisms to anti-angiogenic treatments on EGFR over-expressing glioblastomas.

Structures of adnectin/protein complexes reveal an expanded binding footprint.

Adnectins are targeted biologics derived from the tenth type III domain of human fibronectin (¹°Fn3), a member of the immunoglobulin superfamily. Target-specific binders are selected from libraries generated by diversifying the three ¹°Fn3 loops that are analogous to the complementarity determining regions of antibodies. The crystal structures of two Adnectins were determined, each in complex with its therapeutic target, EGFR or IL-23. Both Adnectins bind different epitopes than those bound by known monoclonal antibodies. Molecular modeling suggests that some of these epitopes might not be accessible to antibodies because of the size and concave shape of the antibody combining site. In addition to interactions from the Adnectin diversified loops, residues from the N terminus and/or the ß strands interact with the target proteins in both complexes. Alanine-scanning mutagenesis confirmed the calculated binding energies of these ß strand interactions, indicating that these nonloop residues can expand the available binding footprint.

Phase I and pharmacokinetic study of CT-322 (BMS-844203), a targeted Adnectin inhibitor of VEGFR-2 based on a domain of human fibronectin.

PURPOSE: To determine the maximum tolerated dose (MTD), safety, pharmacokinetics, pharmacodynamics, immunogenicity, and preliminary antitumor activity of CT-322 (BMS-844203), a VEGFR-2 inhibitor and the first human fibronectin domain-based targeted biologic (Adnectin) to enter clinical studies. EXPERIMENTAL DESIGN: Patients with advanced solid malignancies were treated with escalating doses of CT-322 intravenously (i.v.) weekly (qw), or biweekly (q2w). Plasma samples were assayed for CT-322 concentrations, plasma VEGF-A concentrations, and antidrug antibodies. RESULTS: Thirty-nine patients completed 105 cycles of 0.1 to 3.0 mg/kg CT-322 i.v. either qw or q2w. The most common treatment-emergent grade 1/2 toxicities were fatigue, nausea, proteinuria, vomiting, anorexia, and hypertension. Grade 3/4 toxicities were rare. Reversible proteinuria, retinal artery, and vein thrombosis, left ventricular dysfunction, and reversible posterior leukoencephalopathy syndrome were dose limiting at 3.0 mg/kg. The MTD was 2 mg/kg qw or q2w. CT-322 plasma concentrations increased dose proportionally. Plasma VEGF-A levels increased with dose and plateaued at 2 mg/kg qw. Anti-CT-322 antibodies developed without effects on pharmacokinetics, VEGF-A levels, or safety. Minor decreases in tumor measurements occurred in 4 of 34 evaluable patients and 24 patients had stable disease. CONCLUSIONS: CT-322 can be safely administered at 2 mg/kg i.v. qw or q2w and exhibits promising antitumor activity in patients with advanced solid tumors. The absence of severe toxicities at the MTD, demonstration of plasma drug concentrations active in preclinical models, and clinical pharmacodynamic evidence of VEGFR-2 inhibition warrant further development of CT-322 and suggest strong potential for Adnectin-based targeted biologics.

Adnectin CT-322 inhibits tumor growth and affects microvascular architecture and function in Colo205 tumor xenografts.

Antiangiogenesis has become a promising pillar in modern cancer therapy. This study investigates the antiangiogenic effects of the PEGylated Adnectin™, CT-322, in a murine Colo-205 xenograft tumor model. CT-322 specifically binds to and blocks vascular endothelial growth factor receptor (VEGFR-2). Adnectins are a novel class of targeted biologics engineered from the 10th domain of human fibronectin. CT-322 treated tumors exhibited a significant reduction in tumor growth of 69%, a 2.8 times lower tumor surface area and fewer necrotic areas. Control tumors showed a 2.36-fold higher microvessel density (MVD) and a 2.42 times higher vessel volume in corrosion casts. The vascular architecture in CT-322-treated tumors was characterized by a strong normalization of vasculature. This was quantified in corrosion casts of CT-322 treated tumors in which the intervascular distance (a reciprocal parameter indicative of vessel density) and the distance between two consecutive branchings were assessed, with these distances being 2.21 times and 2.37 times greater than in controls, respectively. Fluorescence molecular tomography (FMT) equally affirmed the inhibitory effects of CT-322 on tumor vasculature as indicated by a 60% reduction of the vascular probe, AngioSense, accumulating in tumor tissue, as a measurement of vascular permeability. Moreover, AngioSense accumulation was reduced as early as 24 h after starting treatment. The sum of these effects on tumor vasculature illustrates the anti-angiogenic mechanism underlying the antitumor activity of CT-322 and provides support for further evaluation of this Adnectin in combinatorial strategies with standard of care therapies.

A fibronectin scaffold approach to bispecific inhibitors of epidermal growth factor receptor and insulin-like growth factor-I receptor.

Engineered domains of human fibronectin (Adnectins™) were used to generate a bispecific Adnectin targeting epidermal growth factor receptor (EGFR) and insulin-like growth factor-I receptor (IGF-IR), two transmembrane receptors that mediate proliferative and survival cell signaling in cancer. Single-domain Adnectins that specifically bind EGFR or IGF-IR were generated using mRNA display with a library containing as many as 10 ( 13) Adnectin variants. mRNA display was also used to optimize lead Adnectin affinities, resulting in clones that inhibited EGFR phosphorylation at 7 to 38 nM compared to 2.6 µM for the parental clone. Individual, optimized, Adnectins specific for blocking either EGFR or IGF-IR signaling were engineered into a single protein (EI-Tandem Adnectin). The EI-Tandems inhibited phosphorylation of EGFR and IGF-IR, induced receptor degradation, and inhibited down-stream cell signaling and proliferation of human cancer cell lines (A431, H292, BxPC3 and RH41) with IC 50 values ranging from 0.1 to 113 nM. Although Adnectins bound to EGFR at a site distinct from those of anti-EGFR antibodies cetuximab, panitumumab and nimotuzumab, like the antibodies, the anti-EGFR Adnectins blocked the binding of EGF to EGFR. PEGylated EI-Tandem inhibited the growth of both EGFR and IGF-IR driven human tumor xenografts, induced degradation of EGFR, and reduced EGFR phosphorylation in tumors. These results demonstrate efficient engineering of bispecific Adnectins with high potency and desired specificity. The bispecificity may improve biological activity compared to monospecific biologics as tumor growth is driven by multiple growth factors. Our results illustrate a technological advancement for constructing multi-specific biologics in cancer therapy.

Anti-tumor effect of CT-322 as an adnectin inhibitor of vascular endothelial growth factor receptor-2.

CT-322 is a new anti-angiogenic therapeutic agent based on an engineered variant of the tenth type III domain of human fibronectin, i.e., an Adnectin™, designed to inhibit vascular endothelial growth factor receptor (VEGFR)-2. This PE Gylated Adnectin was developed using an mRNA display technology. CT-322 bound human VEGFR-2 with high affinity (K(D), 11 nM), but did not bind VEGFR-1 or VEGFR-3 at concentrations up to 100 nM, as determined by surface plasmon resonance studies. Western blot analysis showed that CT-322 blocked VEGF-induced phosphorylation of VEGFR-2 and mitogen-activated protein kinase in human umbilical vascular endothelial cells. CT-322 significantly inhibited the growth of human tumor xenograft models of colon carcinoma and glioblastoma at doses of 15-60 mg/kg administered 3 times/week. Anti-tumor effects of CT-322 were comparable to those of sorafenib or sunitinib, which inhibit multiple kinases, in a colon carcinoma xenograft model, although CT-322 caused less overt adverse effects than the kinase inhibitors. CT-322 also enhanced the anti-tumor activity of the chemotherapeutic agent temsirolimus in the colon carcinoma model. The high affinity and specificity of CT-322 binding to VEGFR-2 and its anti-tumor activities establish CT-322 as a promising anti-angiogenic therapeutic agent. Our results further suggest that Adnectins are an important new class of targeted biologics that can be developed as potential treatments for a wide variety of diseases.

In vitro antiviral activity of the novel, tyrosyl-based human immunodeficiency virus (HIV) type 1 protease inhibitor brecanavir (GW640385) in combination with other antiretrovirals and against a panel of protease inhibitor-resistant HIV.

Brecanavir, a novel tyrosyl-based arylsulfonamide, high-affinity, human immunodeficiency virus type 1 (HIV-1) protease inhibitor (PI), has been evaluated for anti-HIV activity in several in vitro assays. Preclinical assessment of brecanavir indicated that this compound potently inhibited HIV-1 in cell culture assays with 50% effective concentrations (EC(50)s) of 0.2 to 0.53 nM and was equally active against HIV strains utilizing either the CXCR4 or CCR5 coreceptor, as was found with other PIs. The presence of up to 40% human serum decreased the anti-HIV-1 activity of brecanavir by 5.2-fold, but under these conditions the compound retained single-digit nanomolar EC(50)s. When brecanavir was tested in combination with nucleoside reverse transcriptase inhibitors, the antiviral activity of brecanavir was synergistic with the effects of stavudine and additive to the effects of zidovudine, tenofovir, dideoxycytidine, didanosine, adefovir, abacavir, lamivudine, and emtricitabine. Brecanavir was synergistic with the nonnucleoside reverse transcriptase inhibitor nevirapine or delavirdine and was additive to the effects of efavirenz. In combination with other PIs, brecanavir was additive to the activities of indinavir, lopinavir, nelfinavir, ritonavir, amprenavir, saquinavir, and atazanavir. Clinical HIV isolates from PI-experienced patients were evaluated for sensitivity to brecanavir and other PIs in a recombinant virus assay. Brecanavir had a <5-fold increase in EC(50)s against 80% of patient isolates tested and had a greater mean in vitro potency than amprenavir, indinavir, lopinavir, atazanavir, tipranavir, and darunavir. Brecanavir is by a substantial margin the most potent and broadly active antiviral agent among the PIs tested in vitro.

New, potent P1/P2-morpholinone-based HIV-protease inhibitors.

We have developed efficient synthesis of morpholinone-based cyclic mimetics of the P1/P2 portion of the HIV-1 protease inhibitor Amprenavir. This effort led to discovery of allyl- and spiro-cyclopropyl-P2-substituted inhibitors 17 and 31, both 500 times more potent than the parent inhibitor 1. These results support morpholinones as novel mimetics of the P1/P2 portion of Amprenavir and potentially of other HIV-protease inhibitors, and thus provide a novel medicinal chemistry template for optimization toward more potent and drug-like inhibitors.

Ultra-potent P1 modified arylsulfonamide HIV protease inhibitors: the discovery of GW0385.

A novel series of P1 modified HIV protease inhibitors was synthesized and evaluated for in vitro antiviral activity against wild-type virus and protease inhibitor-resistant viruses. Optimization of the P1 moiety resulted in compounds with femtomolar enzyme activities and cellular antiviral activities in the low nanomolar range culminating in the identification of clinical candidate GW0385.

Novel P1 chain-extended HIV protease inhibitors possessing potent anti-HIV activity and remarkable inverse antiviral resistance profiles.

A novel series of tyrosine-derived HIV protease inhibitors was synthesized and evaluated for in vitro antiviral activity against wild-type virus and two protease inhibitor-resistant viruses. All of the compounds had wild-type antiviral activities that were similar to or greater than several currently marketed HIV protease inhibitors. In addition, a number of compounds in this series were more potent against the drug-resistant mutant viruses than they were against wild-type virus.

Synthesis and antiviral activities of novel N-alkoxy-arylsulfonamide-based HIV protease inhibitors.

A series of novel N-alkoxy-arylsulfonamide HIV protease inhibitors with low picomolar enzyme activity and single digit nanomolar antiviral activity is disclosed.

Optimization of pyrrolidinone based HIV protease inhibitors.

Optimization of P1-substituted pyrrolidinone based HIV protease inhibitors has yielded analogs with very potent antiviral activity.

Inhibition of wild-type and mutant human immunodeficiency virus type 1 proteases by GW0385 and other arylsulfonamides.

The arylsulfonamide derivatives described herein were such potent inhibitors of human immunodeficiency virus type 1 (HIV-1) protease (enzyme, E) that values for the inhibition constants (K(i)) could not be determined by conventional steady-state kinetic techniques (i.e., the minimal enzyme concentration usable for the activity assay was much greater than the value of the dissociation constant). Consequently, two alternative methods were developed for estimation of K(i) values. The first method employed kinetic determinations of values for k(1) and k(-1), from which K(i) was determined (k(-1)/k(1)). The second method was a competitive displacement assay used to determine binding affinities of other inhibitors relative to that of GW0385. In these assays, the inhibitor of unknown affinity was used to displace [(3)H]GW0385 from E.[(3)H]GW0385. From the concentration of E.[(3)H]GW0385 at equilibrium, the concentrations of enzyme-bound and free inhibitors were calculated, and the ratio of the K(i) value of the unknown to that of GW0385 was determined (K(i,unknown)/K(i,GW0385)). The values of k(1) were calculated from data in which changes in the intrinsic protein fluorescence of the enzyme associated with inhibitor binding were directly or indirectly monitored. In the case of saquinavir, the fluorescence changes associated with complex formation were large enough to monitor directly. The value of k(1) for saquinavir was 62 +/- 2 microM(-1) s(-1). In the case of GW0385, the fluorescence changes associated with complex formation were too small to monitor directly. Consequently, the value of k(1) was estimated from a competition experiment in which the effect of GW0385 on the binding of E to saquinavir was determined. The value of k(1) for GW0385 was estimated from these experiments to be 137 +/- 4 microM(-1) s(-1). Because E.[(3)H]GW0385 was stable in the standard buffer at room temperature for greater than 33 days, the value of the first-order rate constant for dissociation of E.[(3)H]GW0385 (k(-1)) could be estimated from the time-course for exchange of E.[(3)H]GW0385 with excess unlabeled GW0385. The value of k(-1) calculated from these data was (2.1 +/- 0.1) x10(-6) s(-1) (t(1/2) = 91 h). The K(i) value of wild-type HIV-1 protease for GW0385, calculated from these values for k(1) and k(-1), was 15 +/- 1 fM. Three multidrug resistant enzymes had K(i) values for GW0385 that were less than 5 pM.

Potent inhibitors of the HIV-1 protease incorporating cyclic urea P1-P2 scaffold.

We have developed synthetic approaches to novel analogues of 2-imidazolidinone scaffold 2, which was found to be an effective P1-P2 mimetic in HIV-1 protease inhibitor 4. This enabled a rapid synthesis of analogues of 4 and subsequently allowed us to evaluate and rationalize the SAR. Accordingly, trans relationship of P1 and P2 substituents in the P1-P2 mimetic, as found in a related 2-pyrrolidone-based scaffold 1, was found necessary for high potency against HIV-1 protease. Results of this study provided further rationale towards subsequent optimization of 2-pyrrolidone-based lead 3, which led us to potent and drug-like HIV-1 protease inhibitors described in a follow-on report (Bioorg. Med. Chem. Lett. 2004, 14, in press. ).

Discovery of potent pyrrolidone-based HIV-1 protease inhibitors with enhanced drug-like properties.

We have developed efficient syntheses of the HIV-1 protease inhibitor 4 and its analogues, which incorporate the pyrrolidone scaffold 2 as P1-P2 moiety. Evaluation of these analogues in the HIV-1 protease enzyme assay resulted in discovery of potent and more water soluble meta-amino- and meta-hydroxy inhibitors 17b and 19b. The SAR observed in this class of PIs could be rationalized with aid of the X-ray structure of inhibitor 28 co-crystallized with the HIV-1 protease, which suggested that the polar meta- (but not para-) benzyl substituents in P2 could side-step the hydrophobic S2 enzyme active pocket by rotating the P2 moiety around its Cbeta-Cgamma bond. Such reorientation allows to engage the unsubstituted, hydrophobic edge of benzyl moiety in P2 in the requisite P2/S2 hydrophobic interaction, and projects polar meta-substituent into the bound water. It appears that the meta-position can be chemically derivatized without potency loss of thus resulting inhibitors, as evidenced by potent 22-26. We thus identified pyrrolidone 2-based inhibitors exemplified by 17b and 19b, which uniquely accommodate both high enzyme potency and which provide a platform for fine-tuning of drug-like properties in this class of PIs by additional chemical manipulations on the meta-position.