NEW IMMUNOSUPPRESSIVE DRUGS

The improved outlook for transplant recipients has followed the introduction of xenobiotic immunosuppressants, that is, those drugs produced by organic synthesis or micro organisms that suppress the immune system. Between 1960 and 1985 only steroids, azathioprine (AZA), and CsA had been adopted for use in clinical transplantation. These were joined by early polyclonal and of late monoclonal anti-T cell antibodies. In the last few years, however, our astonishing progress in understanding the molecular events of the alloresponse has not only made new discoveries possible but also has been in part owing to the xenobiotics and antibodies whose actions help to piece together mechanistic paradigms. With the better knowledge agents have been classified by mechanism of molecular action (Fig. 48-8) (Table 48-2) . [71 ]

CsA inhibits the gene activation necessary for IL-2 production that prevents IL-2R binding and all consequent proliferative events. [72 ] The likely explanation for the CsA effect is that it complexes with its binding intracellular protein cyclophilin and inhibits the function of Ca ²⁺ calcineurin phosphatase. [73 ] As discussed in the events describing the activation of T cells, when calcineurin is not bound to cyclophilin, it cannot function as a serine-thereonine phosphatase to cleave a phosphorous from the DNA binding protein (NFAT), leaving this molecule inactive. Inactive NFAT-P cannot engage the promoter sequence of the IL-2 gene. CsA was widely embraced as the central component for effective multidrug immunosuppression until FK 506 (tacrolimus) was introduced to patients in Pittsburgh in 1988. Tacrolimus, like CsA, combines with a cytosolic protein (FK binding protein) and complexes with Ca ²⁺ activated calcineurin to prevent its phosphatase function. [73 ] , [74 ] Tacrolimus has proven to be of at least equal clinical potency in heart and lung transplant patients and is very useful as a switch from CsA-based immunosuppression when rejection on the latter regimen is refractory. [75 ] , [76 ] The reason for the effectiveness of tacrolimus over CsA for a few patients with refractory rejection is interesting and must relate to the pharmacokinetics of the drug rather than to mechanism of action as both drugs are similar in that regard. [77 ] It would appear that tacrolimus is associated with less facial disfigurement and hirsutism common in those recipients receiving CsA but tends to have equal nephrotoxicity and perhaps a greater neurotoxicity than observed in patients receiving CsA.

Rapamycin (serolimus) (RPM) is a microbial by-product of the actinomyces Streptomyces hygroscopicus that was first found on Easter Island. [78 ] Although serolimus is structurally similar to tacrolimus and also must bind with FK binding protein to be effective, it does not inhibit the calcium-activated calcineurin. [74 ] , [79 ] In fact by competing for FK binding protein, serolimus is theoretically antagonistic to tacrolimus in the inhibition of IL-2. [80 ] , [81 ] The system is obviously more complex because success has occurred with combinations of tacrolimus and serolimus. [79 ] Whereas the biochemical events responsible for the inhibition of the cytokine action by the serolimus-FK binding protein combination are not well understood, it appears that the likely target is a lipid kinase. [82 ] Elimination of this kinase results in blocking of T cell proliferation that is facilitated through the CD28 pathway or an effect on the interaction between IL-2 with its IL-2R. Serolimus appears to be effective with B cells as well. Its possible benefits in mitigating chronic rejection are likely not only owing to its B cell effect but also owing to its limiting effect on growth factors that stimulate smooth muscle proliferation. [83 ] It has been recently observed that RPM potently inhibits the stimulation of vascular smooth muscle cell DNA synthesis derived from platelet-derived growth factor and basic fibroblast factor in all cultures. [84 ]

RPM is joined by LEFLUNOMIDE (LFM) as an inhibitor of cytokine action. [85 ] Current investigations suggest that its mechanism is related to its blocking activity on tyrosine kinases associated with cell surface growth factor receptors. [86 ] Although this drug has been used in patients with rheumatoid arthritis, much less is known about its potential usefulness in humans. [87 ] Both serolimus and LFM represent potential targets for use with the cytokine inhibitors CsA or tacrolimus. Combinations of these potential drugs that act in both Ca ²⁺ -dependent and independent fashion in limiting the activation sequence of T and B cells likely will present additive immunosuppressive effects. This will not only effectively reduce the alloresponse but also potentially do so with lower doses of each.

Antimetabolites are immunosuppressive because they inhibit the synthesis of nucleotides necessary for DNA's rapidly dividing cells. The classic drug for the postcytokine active antimetabolites has been AZA, which inhibits purine synthesis throughout all bone marrow lineages. Three new inhibitors of DNA synthesis have undergone evaluation and are at various stages of clinical investigation. The first two, mycophenolate mofetil (MMF) and mazorabine (MZR), appear to be more selective for T and B cells than classic antimetabolite AZA. [88 ] – [90 ] Their selectivity is based on their ability to block the activity of enzyme inosine monophosphate dehydrogenase, which is responsible for the synthesis of purines in the “ de novo ” pathway. Unlike other parenchymal and other peripheral blood cells, T cells and B cells cannot use the “alternative” or salvage pathway for purine synthesis. Panneutropenia has not been a limiting factor with these drugs as it has been with AZA. Brequinar sodium (BQR) is the third new addition to the antimetabolite group. [91 ] , [92 ] Unlike the others, its action appears to be directed against an enzyme in the pathway leading to synthesis of pyrimidines. Less selective than MZR or MMF, BQR has been shown to be myelotoxic, and its rationale for use as an immunosuppressant is dependent on the assumption that immune cells are more sensitive to its antimetabolic effects than nonimmune cells. It is similar to AZA in this regard. Although clinical development currently is on hold, this drug likely will emerge should the previous two in this category fail current expectations.

The last of the new xenobiotics is deoxyspergualin (DSG). [93 ] Because DSG is not an effective inhibitor of cytokine synthesis or of their actions or of DNA synthesis, its immunosuppressant mechanism likely is linked to its inhibition of cell maturation of T and B cells. [94 ] , [95 ] It also acts on APC function. Clinical trials are being conducted in the United States in high-risk renal allograft recipients who previously have undergone one or two allografts within the year. DSG is limited because it must be administered intravenously and is being used as an induction course in conjunction with conventional immunosuppression. Broadened use will depend on the early trials.

Throughout the last 25 years various preparations have been used as anti-T cell serum. In fact, in Pittsburgh, in support of our first cardiac transplant in 1968, equine-antibody-to-human-thymus was made on Dr. Henry T. Bahnson's small farm. Commercially available serum has standardized the product to some extent. These polyclonal anti-T cell preparations recognize T-cell surface structures and kill these targets by inducing FC-receptor mediated cell lysis or by complement-dependent cell lysis. When the monoclonal antibody preparations were first made available in the mid-1980s, transplant physicians were given a standardized product with which to treat the alloresponse. [96 ] The most commonly used monoclonal antibody has been the anti-CD3 antibody (OKT3) that recognizes the epsilon protein of the CD3 complex. Although it was assumed initially that the binding of the antibody would expose its then-activated FC piece to cytotoxic cells that in turn would remove the coated T cell from the circulation, further evaluation has demonstrated that this is likely to be only a minor component of the effectiveness of the antibodies. It has been learned that the major mechanism of anti-CD3 action is to coat the surface TCR—CD3 complex with antibody, causing it to be internalized or shed from the cell surface. The lymphocyte without a TCR—CD3 complex, although present, no longer is active in the alloresponse. Experience in heart and lung transplantation has shown that the commercially available anti-CD3 antibody is effective in eliminating acute rejection, but its role as a prophylactic agent to improve long-term survival has been challenged. Multiple other monoclonal antibodies, including those to CD4, ICAM-1, TCR, LFA-1, and IL-2 receptors, are in various stages of clinical development. It can be assumed that a few of these, if not all of them, will have a specific role in therapy, but hopes for a magic bullet likely will not be realized, as the immune response is far from simple and is based on redundancy by way of alternative pathways. Research will focus on antibodies that could selectively eliminate or activate alloreactive T cells without enabling the host defense against usual pathogens.