I. First-Line Maintenance Immunosuppression

Maintenance regimens combine agents with complementary mechanisms to prevent both acute and chronic rejection while carefully balancing toxicity. The cornerstone of modern therapy includes calcineurin inhibitors, antiproliferative agents, and corticosteroids.

1. Calcineurin Inhibitors (CNIs)

• Mechanism: Inhibit calcineurin phosphatase, which prevents the dephosphorylation of Nuclear Factor of Activated T-cells (NFAT). This blockade reduces IL-2 gene transcription, a critical step in T-cell activation.
• Indications: Backbone of maintenance immunosuppression in kidney, lung, and heart transplantation. Tacrolimus is generally favored due to lower rejection rates observed in registry data.
• Dosing & PK/PD in Critical Illness:
  • Tacrolimus: IV 0.01–0.05 mg/kg/day (continuous infusion) or PO 0.05–0.1 mg/kg/day in two divided doses. Target trough (C0): 5–12 ng/mL. The typical IV-to-PO conversion ratio is 1:3.
  • Cyclosporine: IV 3–5 mg/kg/day or PO 5 mg/kg/day in two divided doses. Target trough (C0): 150–300 ng/mL. The typical IV-to-PO conversion ratio is 1:2.
  • In Critical Illness: Alterations such as decreased albumin (increasing free fraction), increased volume of distribution (Vd), and variable CYP3A4 activity necessitate a “start low, go slow” approach, titrating aggressively based on trough levels and clinical status.
• Monitoring: Trough levels twice weekly until stable, then weekly. Monitor renal function (SCr, electrolytes), blood pressure, and for signs of neurotoxicity.
• Contraindications/Warnings: Known hypersensitivity and uncontrolled active infections. Be vigilant for drug interactions: strong CYP3A4 inhibitors (e.g., azole antifungals, macrolides) increase CNI levels, while inducers (e.g., phenytoin, rifampin) decrease them.

2. Antiproliferatives

• Mechanism: Mycophenolate mofetil (MMF) is hydrolyzed to mycophenolic acid (MPA), which inhibits inosine monophosphate dehydrogenase, a key enzyme in de novo purine synthesis, thus blocking lymphocyte proliferation. Azathioprine (AZA) is a prodrug of 6-mercaptopurine (6-MP), which incorporates into nucleic acids and halts cell division.
• Dosing & Organ Dysfunction:
  • MMF: 1 g PO BID. Reduce dose to 500 mg BID if eGFR < 30 mL/min to avoid accumulation of the inactive but potentially toxic metabolite, MPAG.
  • AZA: 1–3 mg/kg/day PO. Reduce dose in patients with thiopurine methyltransferase (TPMT) deficiency or significant hepatic impairment.
• Monitoring: Complete blood count (CBC) weekly initially, then monthly. Hold for leukopenia (WBC < 3,000/mm³). Assess for GI intolerance (diarrhea), a common side effect of MMF.

3. Corticosteroids

• Mechanism: Broad anti-inflammatory effects via inhibition of proinflammatory gene transcription (e.g., cytokines, chemokines) and reduction of leukocyte trafficking to the allograft.
• Pulse Dosing for Acute Rejection: Methylprednisolone 10–15 mg/kg (typically 500-1000 mg) IV daily for 3 days, followed by a transition to a high-dose oral prednisone taper.
• Taper & Monitoring: Taper oral prednisone by approximately 50% every 5–7 days towards a maintenance dose of < 5 mg/day. Monitor for hyperglycemia, hypertension, and mood changes. For long-term use (>3 months), initiate prophylaxis for bone loss with calcium, vitamin D, and consider bisphosphonates.

II. Induction and Second-Line/Adjunctive Therapies

Induction agents and adjunctive therapies are employed to manage heightened immunologic risk at the time of transplant or to treat refractory rejection. These potent agents carry a higher risk of toxicity and are significantly more costly.

1. Basiliximab and Rabbit Antithymocyte Globulin (rATG)

• Basiliximab: A chimeric monoclonal antibody that blocks the IL-2 receptor (CD25) on activated T-cells. Dosed at 20 mg IV on transplant day 0 and day 4.
• rATG: A polyclonal anti-T-cell immunoglobulin derived from rabbits. Dosed at 1.5 mg/kg/day IV for 3–7 days for induction or rejection treatment.
• Monitoring: Daily CBC to monitor for profound lymphopenia, neutropenia, and thrombocytopenia. Premedicate with corticosteroids, acetaminophen, and diphenhydramine to mitigate infusion-related reactions (e.g., fever, rigors, hypotension).

2. Plasmapheresis + IVIG for Antibody-Mediated Rejection (AMR)

• Indications: Diagnosis of AMR requires a triad of: 1) evidence of graft dysfunction, 2) presence of donor-specific antibodies (DSAs), and 3) characteristic histologic injury on biopsy (e.g., C4d staining).
• Protocol:
  • Plasmapheresis (PLEX): 1–1.5 plasma volumes per session for 3–5 sessions, typically on alternate days, to physically remove circulating antibodies.
  • IVIG: Low-dose (100-200 mg/kg) or high-dose (1-2 g/kg) IV post-exchange to provide passive immunity and modulate the immune response.
• Monitoring: Monitor IgG levels pre- and post-IVIG. Closely watch volume status. Time IVIG administration to occur after PLEX to avoid its immediate removal.

3. Rituximab and Complement Inhibitors

• Rituximab (anti-CD20): A monoclonal antibody that depletes B-cells. Standard dose is 375 mg/m² IV weekly for 4 doses. Premedicate to reduce infusion reactions.
• Eculizumab (anti-C5): A monoclonal antibody that blocks terminal complement activation. Dosed at 900 mg IV weekly for 4 weeks, then 1,200 mg every 2 weeks. Requires meningococcal vaccination at least 2 weeks prior to initiation due to risk of encapsulated bacterial infections.
• Safety: Monitor for infectious complications, particularly neutropenia with rituximab and meningococcal sepsis with eculizumab.

III. Dose Adjustment in Special Populations

Organ dysfunction and extracorporeal therapies significantly alter drug clearance in critically ill patients. Dosing must be adjusted based on therapeutic drug monitoring (TDM) and core pharmacokinetic principles.

A. Renal Replacement Therapy (RRT)

• CNIs and Biologics: Tacrolimus, cyclosporine, and large-molecule biologics (e.g., rATG, rituximab) are not significantly removed by intermittent hemodialysis or continuous RRT. Standard dosing should be used, guided by trough levels.
• MMF: The inactive metabolite (MPAG) accumulates in severe renal impairment and is not well-cleared by dialysis. To mitigate potential toxicity, the MMF dose is typically reduced in patients with an eGFR < 30 mL/min.

B. Hepatic Dysfunction

• CNIs: Tacrolimus and cyclosporine are extensively metabolized by hepatic CYP3A4. In patients with moderate to severe hepatic dysfunction (Child-Pugh Class B–C), start CNIs at 50% of the standard dose and titrate cautiously based on frequent trough monitoring.
• AZA and Corticosteroids: Dose reduction is recommended for azathioprine and may be necessary for corticosteroids in patients with severe hepatic impairment.

C. Drug–Drug Interactions in Critical Care

The ICU environment is rife with potential drug-drug interactions. The most critical involve the CYP3A4 pathway affecting CNI metabolism. Frequent TDM is the primary strategy to mitigate risk.

• CYP3A4 Inhibitors (Increase CNI levels): Azole antifungals (voriconazole, posaconazole), macrolide antibiotics (erythromycin, clarithromycin), diltiazem.
• CYP3A4 Inducers (Decrease CNI levels): Anticonvulsants (phenytoin, carbamazepine), rifampin.

IV. Route of Administration and Delivery Devices

Gastrointestinal dysfunction and specific drug properties dictate the choice between IV and enteral administration. Compatibility with feeding tubes is essential for accurate dosing.

A. IV vs. Enteral Conversion

• Tacrolimus: IV-to-PO conversion ratio is approximately 1:3 due to poor oral bioavailability.
• Cyclosporine: IV-to-PO conversion ratio is approximately 1:2.
• When to Use IV: IV administration is preferred in patients with vasopressor-dependent shock, severe GI edema, ileus, or prolonged NPO status to ensure reliable drug delivery.

B. NG/NJ Tube Administration

• MMF: Capsules can be opened and the contents suspended in water for tube administration.
• Tacrolimus: Granule formulations (Envårsus XR, Astagraf XL) are preferred over crushing immediate-release capsules for creating a suspension.
• General Caution: Avoid crushing any enteric-coated or extended-release formulations. Always check institutional protocols or drug information resources for tubing compatibility and flushing requirements.

V. Monitoring and Pharmacoeconomics

A systematic approach involving regular TDM, graft function assessment, and safety surveillance is crucial to optimize outcomes. The high cost of immunosuppressants must be balanced against their efficacy and monitoring burden.

A. Therapeutic Drug Monitoring (TDM)

• Tacrolimus Trough (C0): Target 5–12 ng/mL for most indications.
• Cyclosporine Trough (C0): Target 150–300 ng/mL.
• Frequency: Twice weekly in the immediate post-transplant period or during critical illness until stable, then weekly, and eventually less frequently in the outpatient setting.

B. Efficacy Endpoints

• Primary: Biopsy-proven acute rejection (BPAR) rates.
• Secondary: Graft function markers (e.g., serum creatinine/CrCl for kidney, spirometry/FEV1 for lung, ejection fraction for heart), graft survival, and patient survival.

C. Safety Surveillance

• Infections: Routine screening for opportunistic infections, including CMV PCR and EBV viral load.
• Malignancy: Regular skin exams for non-melanoma skin cancer, the most common post-transplant malignancy.
• Metabolic: Monitoring of blood pressure, lipids, and glucose.

D. Cost vs. Monitoring Burden

• Tacrolimus generally has a higher acquisition cost than cyclosporine but may reduce overall healthcare expenditures by lowering the incidence of costly rejection episodes.
• High-cost adjunctive therapies like IVIG, rituximab, and eculizumab should be reserved for confirmed, refractory rejection to ensure cost-effectiveness.