Rho(D) Immune Globulin

Mechanism of Action

The mechanism of action for patients receiving RhIG in immune thrombocytopenic purpura treatment is not entirely understood. Prevention of Rh isoimmunization appears to be an immunologic blockade of Fc receptors (RcR) within the reticuloendothelial system (RES); other immunomodulatory effects are also possible. Intravenous infusion of anti-D into an Rh-positive recipient leads to antibody coating of the circulating erythrocytes, which are eventually cleared primarily by the spleen. Clearance of sensitized erythrocytes by the RES results in increased platelet counts and reduced bleeding.[4][5]

The mechanism by which RhIG prevents isoimmunization is not completely understood but is likely to suppress the immune response and antibody formation in Rh-incompatible individuals (Rh-negative exposed to Rh-positive RBCs). If administration occurs within 72 hours of full-term delivery, the incidence decreases from 12% or 13% to 1% or 2%. The chances of isoimmunization drop to less than 1% with the administration schedule at 28 weeks gestation and within 72-hours postpartum.[2][6] 

Administration

Administration of RhIG for prevention of Rh isoimmunization is achieved by either intramuscular (IM) or intravenous (IV) injection, depending on the indication and manufacturer recommendations. During pregnancy, a single dose should be given prophylactically at weeks 26 to 28 and again within 72 hours of delivery of an Rh-positive infant.

For any known or suspected maternal exposure to Rh-positive RBCs or any incompatible Rh-positive blood product received, RhIG should be administered within 72 hours and dosed accordingly to the amount of RBC exposure per manufacturer guidelines. In pregnant women, RhIG should be administered every 12 weeks from the first injection to maintain a sufficient quantity of passively acquired anti-D antibodies.  

Administration of RhIG for immune thrombocytopenic purpura is via IV infusion only.[7] There is a risk of complement system activation when IM-only preparations are given intravenously and should always be avoided in patients with immune thrombocytopenic purpura. Peak serum levels are achieved faster with IV administration; however, the half-life is approximately the same (24 days) for IV and IM administration. Dosages are expressed in micrograms or international units (IUs). The conversion scale is 1 microgram = 5 IUs. The presence of RhIG can persist for months after administration and varies by the individual immune response.[8]

Adverse Effects

The most common adverse effects include[9]:

• Chills
• Headache
• Asthenia
• Fever
• Dizziness
• Infection
• Hypertension
• Vasodilation
• Drowsiness
• Shivering
• Diaphoresis 
• Pallor
• Decreased hemoglobin in patients with immune thrombocytopenic purpura
• Increased lactate dehydrogenase  
• Decreased haptoglobins
• Increased serum bilirubin
• Abdominal and back pain
• Antibody development (positive anti-C antibody test)
• Injection site reaction - erythema, discomfort, mild pain 
• Hyperkinetic muscle activity 

The most serious adverse interactions include[10]:

• Disseminated intravascular coagulation
• Intravascular hemolysis (immune thrombocytopenic purpura patients)
• Acute renal insufficiency
• Clinically compromising anemia
• Death

Contraindications

The FDA-labeled contraindications for RhIG vary by manufacturer but include the following:

1. A hemolytic reaction may result if given to Rh-positive patients; RhIG should be avoided in these individuals. 
2. Patients with a history of severe hypersensitivity or anaphylaxis to any human immune globulin product.
3. Patients with preexisting hemolysis, autoimmune hemolytic anemia, or high risk for hemolysis.
4. RhIG products may contain a small quantity of IgA and should be avoided in IgA deficient individuals with antibodies against IgA.

Monitoring

According to published monitoring parameters on Rh(D) immune globulin, if indicated for the alloimmunization prophylaxis, after administration, patients should be monitored for at least 20 minutes to watch for any signs of systemic reactions.  

If using RhIG to treat immune thrombocytopenic purpura, the black-box warning recommends closely monitoring patients for at least 8 hours to rule out any evidence of intravascular hemolysis. While most patients will be asymptomatic, signs and symptoms to be mindful of in a minority of patients include fever, back pain, shaking chills, and discolored urine or hematuria, which may indicate hemolysis.[10]

Recommended labs for monitoring before treatment initiation include complete blood count (baseline anemia assessment and degree of hemolysis), differential and PBS review (evidence of hemolysis, infection), direct antiglobulin test and antibody screen, reticulocyte count, urinalysis, serum creatinine, and BUN.[10]

Recommended labs to monitor following treatment include complete blood count (1 to 3 days after infusion if first administration), urinalysis 1 to 2 hours after treatment (dipstick only, unless positive for blood), serum creatinine (only if posttreatment hemoglobin decreased greater than 1 g/dL), and BUN (only if posttreatment hemoglobin decreased greater than 1 g/dL).[10]

If intravascular hemolysis is suspected, monitor plasma hemoglobin, LDH, haptoglobin, and plasma bilirubin (direct and indirect).  

Toxicity

Severe and life-threatening toxicities are rare with RhIG treatments. In the event of an overdose in immune thrombocytopenic purpura, patients should be closely monitored to check that the hemoglobin doesn't decrease by more than 1.2 g/dL.[11]

In the suppression of Rh isoimmunization, hemolytic reactions can occur in the case of incompatible blood transfusions after very large doses of RhIG.

Patients should be monitored closely for signs and symptoms of hemolysis with the initiation of supportive treatment at the start of symptoms.

Enhancing Healthcare Team Outcomes

Several RhIG products are currently available in the United States. Although there are multiple brand names, they all have similar clinical indications and dosing but differ significantly in specific attributes. Pharmacists can play an essential role in ensuring the safe administration of these products by guiding clinicians on the use and timing of RhIG therapy. This is one example of the kind of interprofessional activity that must accompany RhIG therapy, with clinicians, mid-level practitioners, specialists, nurses, and pharmacists working and communicating collaboratively to achieve optimal patient outcomes while minimizing adverse effects. [Level 5]

The recommendation is to delay live vaccine administration until at least 12 weeks after the last dose of RhIG. However, according to CDC guidelines, postpartum vaccination of rubella-susceptible women with rubella or MMR vaccine should not be delayed during the last trimester or delivery. It should be serologically tested 6 to 8 weeks post-vaccination to ensure that seroconversion has occurred.

If RhIG is inadvertently omitted after delivery, it should be given as soon as possible within the first 72 hours. Partial protection can be achieved when administered with 13 days of birth and may provide some benefit as much as 28 days after delivery; the longer the delay, the less protective.[12][13] [Level 2] [Level 3]