Entry inhibitors for prevention of hepatitis C virus infection in organ transplantation

Abstract

Entry inhibitors are emerging as an attractive class of therapeutics for hepatitis C virus (HCV) infection. Entry inhibitors target either virion-associated factors or cellular factors necessary for infection. By blocking entry into cells, entry inhibitors prevent both the establishment of persistent reservoirs and the emergence of resistant variants during viral replication. Furthermore, entry inhibitors protect naïve cells from virus-induced alterations. Combining entry inhibitors with direct acting antivirals (DAAs) may therefore improve treatment outcomes, particularly in the context of organ transplantation. The role of DAAs in transplantation is still under clinical investigation, yet carries the risk of recipient infection and HCV-induced disease, since DAAs act only after infection is established. Using entry inhibitors to block infection in transplanted individuals will likely improve patient outcomes during organ transplantation. This may also open perspectives for transplant of organs from HCV-positive donors to HCV-negative recipients to alleviate the medical burden of organ shortage.

Approximately 150 million people are chronically infected with hepatitis C virus (HCV) worldwide. Due to the asymptomatic nature of the disease, many chronically infected individuals develop severe liver disease, including fibrosis, cirrhosis and hepatocellular carcinoma (HCC), in the absence of treatment. As there is currently limited screening and access to treatment,¹ HCV-associated disease burden will likely remain considerable, even in the era of effective direct-acting antivirals.² Currently, HCV-induced end-stage liver disease is the leading indication for liver transplantation (LT).³ In the United States, for example, approximately 40% of LT is due to liver failure associated with HCV infection. However, transplant of an HCV-negative donor liver into an HCV-positive recipient universally leads to graft reinfection in the absence of treatment. Allograft failure due to reinfection is the most common cause for retransplantation and death among HCV-infected LT recipients.

Despite the remarkable cure rates achieved by state-of-the-art direct-acting antivirals (DAAs), their ability to prevent liver graft reinfection still remains to be fully determined.^3–5 For example, 35% of patients experienced liver graft reinfection even when treated with sofosbuvir and ribavirin during LT.⁶ Although DAA-based treatment regimens appear to be effective for some patients undergoing transplant, the high costs of even a single course of DAA therapy pose a barrier to widespread use.⁷ Current DAAs act after infection is already established, when virus-induced disease and drug-related adverse effects pose potential risks for the patient, especially in the context of LT where patients undergoing transplant are immunosuppressed and may present with comorbidities. Furthermore, HCC risk persists even following viral cure, and enhanced HCC recurrence associated with use of DAA in several cohorts is under investigation.⁸ Finally, although drug resistance in DAA combination therapies is limited to a small fraction of patients, it can still arise, and some patient groups may pose particular challenges.⁹ Overall, the current strategy of DAA treatment following LT, while successful in the large majority of treated patients, does pose a number of risks.

In general, a major challenge facing transplantation is organ shortage. In the United States, there are currently approximately 120,000 patients on waiting lists for organ transplant, yet only approximately 30,000 transplants are performed annually.¹⁰ The most acute need is for kidneys, where the median wait time for a kidney transplant is 4.5 years,¹¹ and 13 people are estimated to die each day waiting for a kidney transplant.¹⁰ Organ shortage could be alleviated by transplanting organs from HCV-positive patients into HCV-negative recipients, which no longer poses a significant contraindication.^12–14 Shorter waitlist times where HCV-positive organs are accepted lead to improved transplant outcomes.¹⁵ Nonetheless, transplant recipients receiving organs from HCV-positive donors are at risk of acquiring HCV infection following transplantation. In an early study before DAA therapy was available, 50% of recipients of organs (kidney, heart and liver) transplanted from HCV-positive donors became HCV-positive.¹⁶ Even with DAA treatment, transplantation of organs from HCV-positive individuals to HCV-negative recipients is often associated with less positive outcomes,¹² which is likely attributed to de novo HCV infection of the recipient. Furthermore, the high costs associated with DAA therapy warrant the pursuit of more cost-effective measures.⁷ Novel strategies to address these limitations could increase the pool of donor organs available, improve transplant outcomes and reduce costs to further alleviate the medical burden of organ shortage.

One such strategy is to block de novo infection altogether by using HCV entry inhibitors. In the case of LT for individuals with HCV-induced end-stage liver disease, this would protect the transplanted liver from both infection and potential virus-induced complications. Notably, this is the standard of care in LT for hepatitis B virus (HBV) infection,¹⁷ where it is critical to block the establishment of HBV reservoirs in the host cell nucleus (i.e., persistent covalently-closed circular DNA or viral integration). Although HCV does not establish persistence by these mechanisms, similar approaches to prevent infection of transplanted organs should be considered for HCV. This would allow HCV-positive patients to receive liver transplants from HCV-negative donors without the risk of graft reinfection. Similarly, for HCV-negative recipients receiving organs from HCV-positive donors, entry inhibitors could be used to protect the recipient from HCV infection and potential virus-induced disease, and avoid costs associated with DAA therapy.

HCV entry is a multi-step process that involves several cellular factors and as such provides a number of antiviral targets. Following initial attachment of virions to cellular glycosaminoglycans¹⁸ and lipoprotein receptors,¹⁹ interaction with the scavenger receptor class B type I (SR-BI)²⁰ is thought to prime the viral particle for binding to CD81.²¹ CD81 engagement regulates viral entry by activating signalling pathways such as the epidermal growth factor receptor (EGFR) cascade.²² Association of CD81 with the tight junction protein claudin-1 (CLDN1)²³ is required for the internalization of the HCV particle into clathrin-containing endosomes, allowing for low pH-induced fusion. Other factors, such as the Niemann-Pick C1-like 1 (NPC1L1) cholesterol absorption receptor,²⁴ also contribute to HCV entry, although the specific mechanisms involved and their in vivo roles have not been fully elucidated.

A number of inhibitors interfering with HCV entry are in preclinical and clinical development.²⁵ Entry inhibitors offer a number of attractive features. By acting through complementary mechanisms of action, they may synergize with clinically approved DAAs to reduce the duration and cost of treatment.⁹ Furthermore, they block viral replication at a step before persistent reservoirs can be established. Blocking viral entry may also allow cure of chronic HCV infection, as virus-infected hepatocytes are eliminated by hepatocyte turnover in the absence of de novo infection.²⁶ Entry inhibitors target either viral or cellular factors. Monoclonal antibodies or small molecules targeting virion glycoproteins typically interfere with viral attachment to cellular host factors, or block conformational changes in virion glycoproteins required for fusion.²⁵ However, antivirals targeting non-virally encoded features are expected to have a higher genetic barrier to resistance. Molecules targeting HCV envelope lipids (derived from host cell membranes) modulate membrane fluidity or curvature to block fusion of viral and cellular membranes show great promise in preclinical models.²⁵ However, antiviral approaches targeting host proteins are the furthest along in development.

CD81, SRBI and CLDN1 are particularly relevant host targets, considering their well-defined and crucial roles in HCV entry. Indeed, monoclonal antibodies directed against CD81,²⁷ SR-BI,²⁸ and CLDN1^{26, 29} protect human liver chimeric mice from HCV infection by interfering with the HCV entry process. A small molecule inhibitor of SR-BI termed ITX5061 inhibits HCV entry at a post-binding step and was evaluated in a phase 1b clinical trial in liver transplant recipients with HCV infection (NCT01560468). ITX5061 was safe and well tolerated,³⁰ and reduced plasma HCV RNA following transplant.³¹ Other entry inhibitors in clinical evaluation for HCV infection are also small molecules, in this case targeting EGFR (erlotinib; clinical trial NCT01835938) and NPC1L1 (ezetimibe; clinical trial NCT02126137).

Polyclonal immune globulins targeting HCV (analogous to the HBV immune globulin in standard of care in the context of LT for HBV infection) broadly protect against HCV infection in cell culture³² and are effective in human liver chimeric mice.³³ The polyclonal immune globulin Civacir is under evaluation in a phase 3 clinical trial for prevention of graft reinfection (NCT01804829). Broadly neutralizing monoclonal antibodies targeting the HCV envelope block viral entry to prevent and cure infection in cell culture and human liver chimeric mice have also been identified.^{34, 35} Interestingly, one such antibody combined with a single DAA effectively reduced HCV recurrence post-transplant in an exploratory efficacy trial for patients with active HCV infection at time of transplantation.³⁶ This study provides proof of concept for inclusion of entry inhibitors in combination therapy approaches to prevent HCV infection during transplantation.

DAAs have revolutionized HCV treatment, and by doing so have enabled major advances in organ transplantation in HCV-infected individuals. Furthermore, they have opened perspectives to alleviate organ shortage by enabling transplant of organs from HCV-positive individuals to HCV-negative recipients. However, the potential risks of DAA failure⁹ and HCV-induced liver disease,⁸ including the severe variant of fibrosing cholestatic hepatitis, as well as the high costs associated with DAAs,⁷ warrant the investigation of novel complementary approaches to prevent infection in transplanted individuals, rather than relying on treatment of patients after infection is established. HCV entry inhibitors, some of which have shown promise in clinical trials, are poised to augment current therapeutic approaches to improve organ transplantation outcomes where HCV infection is possible. If short-term administration of entry inhibitors peri-transplantation during the hospital stay can efficiently prevent HCV infection, entry inhibitors may not only improve patient outcomes but also simplify management and potentially reduce costs of antiviral therapy. The optimal deployment of such a prophylactic strategy will depend on several important factors, including cost, safety, and convenience. Further evaluation of a variety of prophylactic approaches appears warranted, particularly as transplant centres begin exploring empiric DAA prophylaxis. An ounce of prevention may indeed be worth a pound of cure for HCV infection during transplantation.