Evolving therapeutic paradigms for HIV and HCV

Isolation of scFvs that inhibit the NS3 protease of hepatitis C virus by a combination of phage display and a bacterial genetic screen

The need for inhibitors for enzymes linked with microbial infection, specifically the NS3 protease of hepatitis C virus (HCV), inspired us to develop a unique, rapid and easy color-based method described herein. The NS3 serine protease of HCV has a role in processing viral polyprotein and it has been implicated in interactions with various cell constituents, resulting in phenotypic changes including malignant transformation. NS3 is currently regarded a prime target for antiviral drugs.We established a genetic screen that is based on coexpression of NS3, a beta-galactosidase reporter that is cleavable by NS3, and potential inhibitors within the same bacterial cell. A single-chain antibody (scFv) library was prepared from spleens of NS3-immunized mice and the screen was used to isolate a panel of protease-inhibiting scFvs. Candidate scFvs were validated for inhibitory activity using an o-nitrophenyl-beta-galactoside (ONPG) hydrolysis assay.The methods can be used more generally to isolate protease-inhibiting cytoplasmic intrabodies able to inhibit proteases or other activities that can be linked with the phenotype of Escherichia coli.

Darunavir: in the treatment of HIV-1 infection

Darunavir (TMC114) is a nonpeptidic peptidomimetic HIV protease inhibitor (PI), with a high binding affinity and a close fit within the substrate envelope. Darunavir shows potent in vitro activity against a broad range of clinical isolates of HIV type 1 (HIV-1), including those with decreased susceptibility to most available PIs. The bioavailability of oral darunavir is increased when it is coadministered in combination with low-dose (100mg) ritonavir. In the POWER 1 and POWER 2 trials, two 144-week randomised phase IIb trails, a reduction in plasma HIV-1 RNA levels of > or = 1 log10 copies/mL (primary endpoint) occurred in 77% and 62% of treatment-experienced recipients of darunavir plus ritonavir (darunavir/ritonavir) 600mg/100mg twice daily (in combination with an optimised background regimen) [vs 25% and 14% of control PI (CPI) recipients; p < 0.001] at week 24. Results are from primary analyses (n = 301 and 201). In a pooled subgroup analysis of data from the POWER 1 and 2 trials, reductions in HIV-1 RNA levels of > or = 1 log10 copies/mL were achieved in 61% of patients treated with darunavir/ritonavir 600mg/100mg twice daily verus 15% of CPI recipients (p < 0.0001) at week 48. Darunavir/ritonavir 600mg/100mg was generally well tolerated in the POWER 1 and 2 trials, with a tolerability profile similar to that of comparator CPIs.

HCV NS3 serine protease-neutralizing single-chain antibodies isolated by a novel genetic screen

Hepatitis C virus (HCV) infection is a major world-wide health problem causing chronic hepatitis, liver cirrhosis and primary liver cancer. The high frequency of treatment failure points to the need for more specific, less toxic and more active antiviral therapies for HCV. The HCV NS3 is currently regarded as a prime target for anti-viral drugs, thus specific inhibitors of its activity are of utmost importance. Here, we report the development of a novel bacterial genetic screen for inhibitors of NS3 catalysis and its application for the isolation of single-chain antibody-inhibitors. Our screen is based on the concerted co-expression of a reporter gene, of recombinant NS3 protease and of fusion-stabilized single-chain antibodies (scFvs) in Escherichia coli. The reporter system had been constructed by inserting a short peptide corresponding to the NS5A/B cleavage site of NS3 into a permissive site of the enzyme beta-galactosidase. The resulting engineered lacZ gene, coding for an NS3-cleavable beta-galactosidase, is carried on a low copy plasmid that also carried the NS3 protease-coding sequence. The resultant beta-galactosidase enzyme is active, conferring a Lac+ phenotype (blue colonies on indicator 5-bromo-4-chloro-3-indolyl beta-D-galactoside (X-gal) plates), while induction of NS3 expression results in loss of beta-galactosidase activity (transparent colonies on X-gal plates). The identification of inhibitors, as shown here by isolating NS3-inhibiting single-chain antibodies, expressed from a compatible high copy number plasmid, is based on the appearance of blue colonies (NS3 inhibited) on the background of colorless colonies (NS3 active). Our source of inhibitory scFvs was an scFv library that we prepared from spleens of NS3-immunized mice and subjected to limited affinity selection. Once isolated, the inhibitors were validated as genuine and specific NS3 binders by an enzyme-linked immunosorbent assay and as bone fide NS3 serine protease inhibitors by an in vitro catalysis assay. We further show that upon expression as cytoplasmic intracellular antibodies (intrabodies) in NS3-expressing mammalian cells, three of the scFvs inhibit NS3-mediated cell proliferation. Although applied here for the isolation of antibody-based inhibitors, our genetic screen should be applicable for the identification of candidate inhibitors from other sources.