Thursday, April 26, 2012

Antibody-Mediated Rejection: Choose your weapons

Acute humoral or antibody-mediated rejection (AMR) is attributed to the presence of alloantibodies against the graft, which could be either antibodies against human leukocyte antigens (HLAs) Class I and/or II , non-HLA antigens or endothelial antigens. Diagnosis of AMR is made through tissue biopsy and presence of alloantibodies. Early treatment is of paramount for the preservation of graft function. Treatment strategies include removal of alloantibodies, decreasing or stopping production of alloantibodies, or attenuating the immune systems response to alloantibodies. 

Plasmapheresis/Plasma Exchange 
Removal of alloantibodies is done through the use of plasmapheresis/plasma exchange or immunoadsorption. Plasmapheresis, or removal and replacement of one plasma volume, is effective at removing approximately 60% of the intravascular IgG which accounts for about 75% of the intravascular immune response. Extravascular IgG equilibrates in about 48 hours thus reducing total body IgG concentrations and reducing the effective immune response. Immunoadsorption works similarly to plasmapheresis except that plasma immune complexes and IgG are removed via protein A bound silica matrices. In the latter, the remaining plasma components are returned to the patient without the need for plasma exchange. FFP is needed even on the first run of plasmapheresis if recent biopsy was performed (prevention of bleeding). The cost of 5 treatments is about $4,600. 

Intravenous Immune Globulin 
Infusion of intravenous immunoglobulins (IVIG) has been studied at doses of 10 grams to 2 gm/kg as monotherapy or in conjunction with plasmapheresis or B-cell depleting agents. The mechanism of action is not entirely known but it is thought that neutralization of alloantibodies occurs when bound by the anti-idiotypic antibodies in IVIG as well as diminished plasma cell production by increasing total body concentrations of immunoglobulins and direct T-cell and complement cascade effects. Cost for IVIG is $77.06/gm resulting in $770.60 or $10,788.40 (dose intensity), based on a 70kg patient for each dose. 

Decreasing or stopping the production of alloantibodies requires therapies directed against mature plasma cells, memory B-cells or plasmablasts. Targeting memory B-cells or plasmablasts has a delayed onset of action as this therapy prevents new plasma cells from being formed but does not affect currently active ones. 

A chimeric anti-CD20 monoclonal antibody, is dosed 375 mg/m2 or 1000 mg IV and given for one to two doses. The CD20 receptor is found on the surface of B-lymphocytes, including memory B-cells and immature plasmablasts, but not plasma cells. Rituximab has cytotoxic activity directly reducing B-lymphocyte and antibody levels. Cost for therapy ranges from $4,923.78 for dosing based on normal body surface area to $7,589.64 for a 1000mg dose. 

A proteasome inhibitor, is dosed 1.3 mg/m2 IV and given for four doses on days 1, 4, 8, and 11. Proteasome inhibition prevents protein biosynthesis resulting in apoptosis of the plasma cell and cessation of alloantibody production. Cost per dose based on a normal body surface area is $1,134.27. 

A humanized monoclonal antibody directed against C5, is dosed 600mg to 1200mg IV and administered weekly depending on alloantibody concentrations. Prevention of AMR is mediated by inhibition of membrane attack complex formation and halting activation of the complement cascade. Eculizumab is supplied as a 300mg vial for $6,638.40 or $13,276.80 to $26,553.60 per dose

Increasing the dose of maintenance immunosuppressive agents, including calcineurin inhibitors, antimetabolites, and steroids are also used to attenuate the immune system and help alleviate AMR. With all of the available treatment options, a multimodal approach is usually recommended to maximize chances of preventing graft injury. However, as you might see from the numbers above, careful clinical decision must be based on both efficacy and cost in order to responsibly avoid collapsing our already broken health care system. Instead of each center using its on protocol, our society should get together and perform a randomized trial with those interventions. Though I doubt this will happen any time soon, in particular with all the NIH budget cuts...

David Reardon, PharmD, PGY2 Critical Care Resident
Steve Gabardi, PharmD
Leonardo V Riella MD PhD (editing role)

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