Life threatening SLE is manifested by one or a combination of several organ systems being damaged or dysfunctional. These include proliferative nephritis, neuropsychiatric symptoms, severe thrombocytopenia, hemorrhagic pneumonitis, myocarditis and pancreatitis. The standard treatment for these manifestations has been high doses of corticosteroids (CS) including "pulse steroids" (methylprednisolone 1g IV qd for three days) and/or immunosuppressive treatment with azathioprine or cyclophosphamide. Recently, plasmapheresis has been evaluated as an adjunct to cyclophosphamide, but did not provide any benefit over cyclophosphamide alone. These treatments are frequently ineffective or too toxic with sepsis and opportunistic infection often limiting their use or resulting in the death of the patient. Long term corticosteroids have additional side effects that include but are not limited to hypertension, hyperglycemia, hyperlipidemia, osteoporosis, cataracts, weight gain, myopathy and emotional instability. Cyclophosphamide also has significant side effects such as infertility, teratogenicity, oncogenicity and hemorrhagic cystitis.

The shortcomings of the conventional therapies for severe SLE have emphasized the need for the development of new therapeutic options. Recently, a number of novel approaches have been tried to treat moderate to severe SLE. These new therapeutic approaches have attempted to target specific immune responses or specific immune reactants and thus not producing the broad cytotoxicity, myelosuppression and immunosuppression encountered with the previously employed drug regimens. These novel approaches include:

1. new cytotoxic agents
2. induction of B cell tolerance
3. stem cell restoration
4. inhibition of idiotypic network
5. targeted molecular therapies.

New Cytotoxic Agents

Cytotoxic agents, such as cyclophosphamide, azathioprine and cyclosporine, are reserved for severe SLE, though their use is limited by their multiple side effects Recently, a new purine antagonist, mycophenolate mofetil (MMF), and halogenated adenosine analogues, fludarabine and cladribine, have been tried in the treatment of SLE.

MMF inhibits inosine monophosphate dehydrogenase, an enzyme found predominantly in lymphocytes. As a result, MMF is relatively selective for proliferating (activated) T and B lymphocytes, as opposed to the more generalized cytotoxicity for all proliferating cells exhibited by cyclophosphamide and azathioprine. There are preliminary reports of patients with diffuse proliferative glomerulonephritis (DPGN) refractory to intravenous cyclophosphamide and cyclosporine, who responded to MMF.(1),(2) An open label pilot trial with ten SLE patients who were inadequately controlled with corticosteroids, antimalarials and other immunosuppressive agents showed that MMF at doses of 1.5 to 2 g daily for 6-18 months was effective and safe for patients with moderate and severe lupus.(3) A study of 22 SLE patients, ten of whom had lupus nephritis with nephrotic range proteinuria, treated with MMF 2 g daily also demonstrated improvement in overall lupus activity in the majority of the patients but no remission of the nephritic syndrome.(4) A recent study demonstrated that MMF and prednisolone was as effective as prednisolone and cyclophosphamide, given for six months, followed by prednisolone and azathioprine for six months in patients with diffuse proliferative lupus nephritis.(5)

Recently, halogenated adenosine analogues, fludarabine and cladribine, have been tried in the treatment of SLE. These antileukemic drugs are activated by kinases present mainly in lymphocytes are thus less cytotoxic to other proliferating cells. Fludarabine has been reported as a successful treatment in a patient with SLE in whom dilated cardiomyopathy precluded the use of high doses of corticosteroids.(6) In addition to anecdotal reports, both fludarabine and cladribine have been reported as promising treatment of lupus nephritis in two pilot studies.(7),(8) Both fludarabine and cladribine caused prolonged lymphopenia; several infections occurred but responded to standard antibiotic treatment.

Induction of Specific B Cell Tolerance

Autoantibodies like antibodies produced to foreign antigens are synthesized by activated B lymphocytes. The activation of the B lymphocyte requires both exposure to antigen and costimulation provided by the interaction between receptors on the B cells and ligands on activated T lymphocytes. In the absence of costimulation, the B cells are anergized and are incapable of synthesizing antibodies. Anti-double stranded DNA antibodies are a hallmark of SLE and are associated with renal damage.

A new approach for treating SLE was generated in the early 1990s with the development of LJP 394 -- a tetrameric form of synthetic DNA, which can react directly (i.e., without T lymphocytes) with the antigen receptors of B cells specific for DNA. It was postulated that this agent, by reacting with these cells in the absence of costimulation, would anergize them. A placebo-controlled study in patients with quiescent SLE demonstrated that LJP at a dose of 50 mg, administered intravenously weekly, could reduce anti-double stranded DNA titers by 40%.(9) Subsequently, a multicenter study confirmed that LJP 394 is capable of reducing dsDNA titers but showed a decrease in renal flares only in those patients with high affinity dsDNA antibodies.(10)

Stem Cell Restoration

The immune system is generated from pluripotential cells in the bone marrow known as stem cells. It is commonly thought, that in SLE, there are clones of dysregulated stem cells that give rise to autoreactive T and B lymphocytes. If these stem cells can be destroyed, it is postulated that the disease can be arrested. This hypothesis is the basis for stem cell therapy -- a novel therapeutic approach to the treatment of SLE. It consists of bone marrow ablation by radiation and combination cytotoxic drugs, followed by autologous or allogeneic stem cell transplantation (11),(12),(13),(14)

A dozen patients with severe SLE who have undergone high dose immunosuppression and autologous stem cell transplantaion have been reported as isolated case reports,(15),(16),(17) A recent phase I study of seven lupus patients with severe nephritis, vasculitis, cerebritis and life-threatening cytopenias, who were unresponsive to at least six monthly cycles of intravenous cyclophosphamide, were treated with autologous stem cell transplantation. All seven remained free of active SLE at a median follow-up of 25 months.(18)

Immunoablative therapy alone, without stem cell transplantation, has also been investigated for treatment of autoimmune diseases refractory to other modalities. This approach avoids the costs involved in stem cell harvesting (autologous) or finding appropriately matched donors (allogeneic). So far, a total of 11 patients with severe SLE have been treated with high dose cyclophosphamide (200 mg/kg) with or without anti-thymocyte globulin but without stem cell support. All of the patients had hematopoetic recovery within 20 days. In four patients, treatment was too recent to evaluate; in seven patients, there was a dramatic improvement with four achieving complete remission; and in three patients, a partial response with a significant decrease in their requirements for continuing immunosuppressive treatment.(19)

Inhibition of the Idiotypic Network

Intravenous immunoglobulin (IVIg) is an immunomodulatory agent capable of regulating lupus activity in animal models and humans. The presumed mechanisms of action are multiple, including, but not limited to, Fc receptor blockade, complement regulation, T cell regulation and regulation of the B cell idiotype network.

IVIg has been reported to beneficial in the treatment of arthritis, thrombocytopenia and neuropsychiatric mainifestations of lupus.(20),(21) IVIg has also been reported to be partially effective in the treatment of membranous and membranoproliferative lupus nephritis resistant to cyclophosphamide and prednisone.(22)

Targeted Molecular Therapies

The development of therapies targeted at molecules that modulate various stages of the immunoinflammatory response is a novel paradigm in the treatment of SLE and several strategies have been explored. Currently, the most relevant are the following approaches:

• inhibition of costimulatory signal pathways in T cell--B cell interactions
• manipulation of the complement system
• manipulation of cytokines.

Inhibition of Costimulatory Pathways

In recent years, it has become clear that the secretion of autoantibodies by B cells is a T cell dependent process. Helper T cells and B cells interact through a sequence of costimulatory events that involve interaction between CD40 on B cells and CD40 ligand (CD40L) on T cells, and B7 molecules on B cells and CD 28 molecules on T cells. Interfering with these costimulatory pathways between T and B lymphocytes is an innovative approach to the treatment of SLE. Another T cell membrane component, CTLA4, is a naturally occurring molecule, which competes with CD28 for B7. The interaction of CTLA4 with B7 results in inhibition of B cell activation and antibody production.

A fusion protein generated from CTLA4 and an immunoglobulin heavy chain constant region (CTLA4Ig) blocks the B7/CD28 interaction and, therefore, inhibits T cell activation and subsequent upregulation of B cell immunogolobulin production. CTLA4Ig has been used successfully to block anti-dsDNA antibody production and to suppress autoimmune nephritis in a mouse model of lupus alone(23) or in combination with cyclophosphamide.(24) Based on these experimental data, CTLA4Ig clinical trials in patients with active lupus nephritis are in planning stages.

As mentioned above, the CD40-CD40L interaction also provides costimulation necessary for B cell proliferation and differentiation. CD40 is expressed on B cells, while CD40L is expressed only on activated T cells. The CD40-CD40L interaction is crucial for the production of antibodies to T-cell dependent antigens. There is evidence that, in SLE, CD40L expression on T cells is enhanced and prolonged; and the soluble forms of CD40L are increased and correlate with disease activity.

Based on encouraging results of anti-CD40 antibody treatment in mouse models of lupus, five patients with lupus nephritis underwent treatment with humanized monoclonal anti-CD40L antibody. This treatment caused a 10-200-fold decrease in the frequency of both IgG and IgM anti-DNA antibody secreting B cells in four patients. Suppression of anti-DNA titers to below the level of detection 28-84 days after the last treatment and a 10-fold decrease in the frequency of the clones secreting anti-DNA antibodies in three patients were also noted.(25)

In addition, a humanized monoclonal antibody to CD40L, IDEC-131, has been employed in a phase I study.(26) Patients with moderate SLE were given a single dose IDEC-131 and followed for three months. There were no serious side effects and no anti-IDEC antibodies developed. Also, there was no evidence of T-cell depletion or uncontrolled cytokine release.

Manipulation of the Complement System

Monoclonal antibody to C5

The activation of the complement system plays an important role in organ damage in SLE. Flares of SLE are marked by a decrease in C3 and C4, and an increase in complement split products that have pro-inflammatory properties: C3a, C3d and the C5b-9 membrane attack complex. It follows that complement components represent a potential target for new therapeutic interventions. A single dose, phase I safety study with a humanized antibody to C5 in SLE and rheumatoid arthritis was completed recently, and was shown to be well tolerated up to the maximum dose given of 8 mg/kg.(27)

Manipulation of Cytokines

Anti-IL-10 monoclonal antibody

When resting T helper lymphocytes are exposed to antigen in combination with costimulation they are activated to either a cytotoxic response (TH1) or to a B cell stimulating response (TH2). It is the latter response that appears to predominate in SLE. One of the principle cytokines secreted by the TH2 cells is the cytokine IL10. It has been found, recently, that in SLE there is an increased spontaneous production of IL-10. These observations have prompted a prospective trial of anti-IL-10 monoclonal antibody (anti-IL-10 mAb) in lupus. In this six-month study, the agent appeared to be beneficial in most patients.(28)

Conclusions

Conventional therapies for SLE are broad spectrum and characterized by lack of specificity for particular pathogenic lymphocyte populations or their products. They cause generalized immune suppression and have multiple side effects. Only recently, we have obtained a better understanding of the regulation of the immune system, the molecular mechanisms important in B cell--T cell interactions, cytokines and hormones that are involved in T and B cell responses, B cell activation and B cell tolerance. This new knowledge gave us an opportunity to pursue several different directions to generate novel targeted therapies for SLE. Since the new therapeutic approaches are more selective than the classical ones, they are expected to be more effective and less toxic. However, these efforts only represent the first steps towards exploring these innovative treatments. Future controlled studies will be needed to establish their role in the treatment of SLE.