Efficacy and safety of ABI793, a novel human anti-human CD154 monoclonal antibody, in cynomolgus monkey renal allotransplantation.

Since the first successful kidney transplantation more than 40 years ago, the treatment of end-stage organ failure has dramatically improved. Many organs are now transplanted, the most common of which include (procedures performed in the US in 2001) the kidney (14,000), heart (2,000), liver (5,000) and lung (1,000). Despite these figures transplantation is still somewhat limited, mainly because of rejection which can destroy the tissue soon after transplantation or over a more prolonged time frame. Limitations in the availability of human donor organs also continue to be important.

Host vs graft reaction is the principal mechanism of acute rejection and causes graft destruction days to months after transplantation. This reaction is characterized histologically by mononuclear cellular infiltration of the allograft, with varying degrees of hemorrhage and edema. After resolution of acute rejection, the allograft commonly survives for prolonged periods, even though immunosuppressive drug dosages are reduced to very low levels. Chronic rejection often progresses insidiously despite increased immunosuppressive therapy through a proces related to antibody-mediated damage. The pathologic picture differs from that of acute rejection. The arterial endothelium is primarily involved, with extensive proliferation that may gradually occlude the vessel lumen, resulting in ischemia and fibrosis of the graft.

Immunosuppressants are used to control rejection and are primarily responsible for the success of transplantation. Immunotherapy commonly involves the use of azathioprine beginning at the time of transplantation often combined with low doses of cyclosporine. However, immunosupressants suppress all immunologic reactions, thus making overwhelming infection the leading cause of death in transplant recipients. In addition immunosupressants are associated with severe toxicity.

The most significant complications of drugs used for transplant patients include nephrotoxicity, neurotoxicity, new-onset post-transplant diabetes mellitus, hyperlipidemia, and hypertension. These side effects occur in part because of the ubiquitous expression of the molecular targets of currently used immunosuppresants. New clinical options including therapeutic antibodies are on the horizon and will hopefully offer immunosuppression with fewer adverse effects than current agents.

The recent success of monoclonal antibody therapies such as Genentech's Rituxan and Johnson & Johnson's Remicade are driving further development of this field. MAb therapies brought in revenues of over $5.1 billion in 2003, a figure which should nearly triple over the next five years new MAbs are launched and following the approval of new indications for MAbs already on the market. By 2008, MAbs should account for 32 percent of all revenue in the biotech market.

Administration of blocking antibodies directed against the CD154 molecule leads to long-term graft survival in a nonhuman primate model. Clinical trials with two humanized mouse anti-human CD154 monoclonal antibodies, hu5c8 (Biogen, Cambridge, MA) and IDEC-131 (IDEC, San Diego, CA), were however, suspended after the occurrence of rare thromboembolic complications.

Recently Novartis researchers have reevaluated the therapeutic potential of CD154 antibodies using human immunoglobulin (Ig) transgenic mice to generate a panel of antibodies with specificity for epitopes on the CD154 molecule that differ form the epitope recognized by previous candidates. This approach was adopted to determine whether the cardiovascular side-effects of previous anti-CD154 antibodies could be overcome by targeting other epitopes.

In their recent Transplantation article Walter Schuler et al report a study in which transgenic mice were stimulated with human CD154. Spleen cells were then sampled and immortalized; one cell line was then selected that secreted an anti-human CD154 antibody, ABI793. This antibody was selective for human CD154 and cross-reacted with CD154 from Cynomolgus monkeys. ABI793 prevented the binding of huCD154 to its binding partner CD40. The affinity of ABI793 was similar to that of two mouse anti-human CD154 antibodies (including the non-humanized version of hu5c8) however closer investigation demonstrated that ABI793 recognized a different epitope on CD154 to that of the murine antibodies.

Schuler et al then proceeded to determine the efficacy of the antibodies when used as a monotherapy in life-supporting renal allotransplantation in Cynomolgus monkeys. Untreated recipients show graft survival of 8 to 9 days. When ABI793 was administered on the day of transplantation, immediately before graft reperfusion, and then on posttransplantation days 1, 4, 11,18, 28, 56, and finally on day 84, median graft survival was 108 days. In five of the nine recipients, graft function was maintained for several weeks after cessation of ABI793 treatment. Four of the nine recipients suffered severe graft failure while still receiving ABI793 treatment.

During early post-surgery recovery, numerous recipients required blood transfusion or erythropoietin treatment. These complications are seen irrespective of the immunomodulatory agents employed. In addition however ABI793-treated Cynomolgus monkeys manifested a decrease in platelet numbers, which occurred independently of the early post-surgery events, differing from those receiving conventional immunosuppression but resembling the thromboembolic complications occasionally seen in the clinic with first generation anti-CD154 antibodies. The decrease in platelet numbers produced chronic and acute thromboembolic vascular lesions and consequent fibrin deposition and hemorrhage. In the present study this was attributed to the use of ABI793 and was reversible following treatment with aspirin. Of interest ABI793 failed to affect platelet number in monkeys subjected to autotransplantation suggesting that this adverse effect was particular to the use of ant-CD154 antibodies in the context of alloantigen recognition.

The cause of the adverse effects seen with anti-CD154 antibody treatment is unknown although it should be noted that CD154 is expressed on activated platelets and binds to platelet integrin. In the absence of CD154 binding large thrombi become destabilized and disintegrate and this may explain the adverse effects. Further clarification of the mechanism behind these adverse will be necessary before the efficacy of molecules such as ABI793 can be exploited in the clinic.

Entry date Sunday, August 29, 2004

Adapted from Schuler et al, Transplantation. 2004 Mar 15;77(5):717-26.