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Mechanism of Action
RhoGAM and MICRhoGAM act by suppressing the immune response of Rh-negative individuals to Rh-positive red blood cells. The mechanism of action is unknown. RhoGAM, MICRhoGAM and other Rho(D) Immune Globulin (Human) products are not effective in altering the course or consequences of Rh immunization once it has occurred.
Use after Rh-Incompatible Transfusion
An Rh-negative individual transfused with one unit of Rh-positive red blood cells has about an 80% likelihood of producing anti-D.4 However, Rh immunization can occur after exposure to < 1 mL of Rh-positive red blood cells. Protection from Rh immunization is accomplished by administering > 20 µg of RhoGAM or MICRhoGAM per mL of Rh-positive red blood cells within 72 hours of transfusion of incompatible red blood cells.13,22
Pharmacokinetic studies after intramuscular injection were performed on sixteen Rh-negative subjects receiving a single dose of (368 µg or 1840 IU) RhoGAM.10 Plasma anti-D levels were monitored for thirteen weeks using a validated Automated Quantitative Hemagglutination method with sensitivity of approximately 1 ng/mL. The following mean pharmacokinetic parameters were obtained from data collected over the first ten weeks of a thirteen-week study:
|Maximum plasma concentration obtained (Cmax)||54.0||13.0||ng/mL|
|Time to attain Cmax (Tmax)||4||days|
|Elimination half-life (T½)||30.9||13.8||days|
|Volume of distribution (Vd)||7.3||1.5||liters|
Rho(D) Immune Globulin (Human) administered at 28 weeks, as well as within 72 hours of delivery, has been shown to reduce the Rh immunization rate to about 0.1-0.2%.23,24 Clinical studies demonstrated that administration of MICRhoGAM within three hours following pregnancy termination was 100% effective in preventing Rh immunization.25
Multiple studies have been performed that prove the safety and efficacy of RhoGAM in both the obstetrical and post transfusion settings.
Freda, Gorman and colleagues26, 27 studied the efficacy of RhoGAM in the postpartum setting in a randomized, controlled study completed in 1967. The control group received no immunoglobulin therapy after delivery, while the test group received 300 µg of RhoGAM intramuscularly within 72 hours of delivery of an Rh-positive infant. Six months after delivery, the incidence of Rh immunization in the control group was 6.4% (32/499) versus 0.13% (1/781) in the RhoGAM group (p < 0.001).
Pollack et al. performed two randomized, placebo-controlled studies in the post transfusion setting that were designed to establish the dose response relationship of RhoGAM. In the first study,15 178 (176 males, 2 females) Rh-negative volunteers received varying volumes of Rh- positive red cells; 92 subjects then received RhoGAM. A single dose of RhoGAM (1.1 mL @ 267 µg/mL) was shown to suppress anti-D formation after injection of up to 15.1 mL of Rh-positive red cells. In a companion study,4 Pollack administered 500 mL of Rh-positive whole blood to 44 Rh-negative male volunteers. Twenty-two (22) subjects received 20 µg RhoGAM per mL of Rh-positive red cells and 22 received no RhoGAM. None of the RhoGAM-treated subjects developed anti-D; 18/22 control arm subjects developed anti-D (p < 0.0001).
Human clinical studies10 were subsequently performed to prove the efficacy of MICRhoGAM and the low protein (5%) formulations. In the MICRhoGAM study, 81 Rh-negative male volunteers received an initial injection of 2.5 mL Rh-positive red cells, followed by a booster injection (0.1 mL) of red cells at 26 weeks; 40 subjects received an injection of MICRhoGAM after the initial red cell injection. None of the subjects who received MICRhoGAM developed anti-D, both before and after the booster red cell injection. A similar study was performed in 1985 using the low protein formulation of RhoGAM. None of the 30 Rh-negative male volunteers who received RhoGAM after injection of 15 mL of Rh-positive red cells developed anti-D.
13 Zipursky A, Israels LG. The pathogenesis and prevention of Rh immunization. Can Med Assoc J 1967;97:1245-56.
22 Crispen J. Immunosuppression of small quantities of Rh-positive blood with MICRhoGAM in Rh-negative male volunteers. In: Proceedings of a symposium on Rh antibody mediated immunosuppression. Raritan, NJ: Ortho Research Institute of Medical Sciences, 1975:51-54.
Last reviewed on RxList: 5/9/2008
This monograph has been modified to include the generic and brand name in many instances.
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