A 31-year-old female receives a kidney transplant for autosomal dominant polycystic kidney disease (ADPKD). Three weeks later, the patient experiences acute, T-cell mediated rejection of the allograft and is given sirolimus. Which of the following are side effects of this medication?

Hyperlipidemia, thrombocytopenia

Nephrotoxicity, gingival hyperplasia

Cytokine release syndrome, hypersensitivity reaction

A 67-year-old woman who was recently diagnosed with Crohn disease comes to the physician for evaluation of her immunosuppressive therapy. She has had recurrent flares since her diagnosis. Physical examination shows two shallow ulcers on her oral mucosa. The physician considers adding azathioprine to her medication regimen. A deficiency of which of the following enzymes would diminish the therapeutic effect of this drug?

Hypoxanthine-guanine phosphoribosyl transferase

Phosphoribosyl pyrophosphate synthetase

A 63-year-old HIV-positive man comes to the physician for a routine health maintenance examination. Four years ago, he was diagnosed with HIV and was started on cART therapy. He tells the physician that he has been having difficulty adhering to his medication regimen. He has been unemployed for the past couple of years and relies on unemployment benefits to cover the costs of daily living. His father died of lymphoma at the age of 60 years. He wants more information about his risk of developing DLBCL. Which of the following is the greatest risk factor for the development of DLBCL in HIV-positive patients?

Positive family history of cancer

A 38-year-old woman comes to the physician for a follow-up examination. Two years ago, she was diagnosed with multiple sclerosis. Three weeks ago, she was admitted and treated for right lower leg weakness with high-dose methylprednisone for 5 days. She has had 4 exacerbations over the past 6 months. Current medications include interferon beta and a multivitamin. Her temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 116/74 mm Hg. Examination shows pallor of the right optic disk. Neurologic examination shows no focal findings. She is anxious about the number of exacerbations and repeated hospitalizations. She is counseled about the second-line treatment options available to her. She consents to treatment with natalizumab. However, she has read online about its adverse effects and is concerned. This patient is at increased risk for which of the following complications?

Progressive multifocal leukoencephalopathy

Syndrome of inappropriate antidiuretic hormone

Immunosuppressant drug monitoring for USMLE Step 1: High-Yield Pharmacology Lessons

TDM Basics - The Balancing Act

Immunosuppressants have a Narrow Therapeutic Index (NTI), where the therapeutic window between efficacy and toxicity is small, making precise dosing critical.
Therapeutic Drug Monitoring (TDM) is used to maintain drug concentrations within this target range, individualizing therapy.

The Balancing Act:
- Goal 1 (Efficacy): Prevent graft rejection by ensuring drug levels are not sub-therapeutic.
- Goal 2 (Safety): Avoid dose-related toxicities (e.g., nephrotoxicity, neurotoxicity) by ensuring levels are not supra-therapeutic.

⭐ For calcineurin inhibitors like Tacrolimus, trough concentrations ($C_0$) are the standard for TDM as they correlate well with total drug exposure ($AUC_{0-12}$).

Calcineurin Inhibitors - Tacro & Cyclo

Calcineurin inhibitors (CNIs) are the backbone of most immunosuppressive regimens, preventing T-cell activation by blocking IL-2 transcription.

Calcineurin Inhibitor Mechanism of Action in Lymphocytes

Feature	Tacrolimus (FK-506)	Cyclosporine
Mechanism	Binds to FKBP-12 to inhibit calcineurin.	Binds to Cyclophilin to inhibit calcineurin.
Key Toxicities	More Neurotoxicity (tremor), Alopecia, New-onset Diabetes.	Hirsutism, Gingival Hyperplasia, Hyperlipidemia.
Shared Toxicity	Nephrotoxicity (vasoconstrictive), Hypertension, Hyperkalemia.	Nephrotoxicity, Hypertension, Hyperkalemia.
Monitoring	Trough (C0) levels.	Trough (C0) levels.
Target Range	Highly variable; typically 5-15 ng/mL.	Highly variable; typically 100-400 ng/mL.

⭐ Exam Favorite: Both are metabolized by CYP3A4. Co-administration with inhibitors (e.g., azole antifungals, macrolides) can ↑ CNI levels and toxicity, while inducers (e.g., rifampin, phenytoin) can ↓ levels, risking rejection.

mTORi & Antimetabolites - Supporting Cast

A table comparing key non-calcineurin inhibitor agents used in immunosuppressive regimens.

Class	Drug(s)	Key Toxicities
mTOR Inhibitors	Sirolimus (Rapamycin), Everolimus	Pancytopenia, poor wound healing, hyperlipidemia, insulin resistance, stomatitis. "Kidney-sparing" (less nephrotoxic than calcineurin inhibitors).
Antimetabolites	Mycophenolate Mofetil	GI distress (esp. diarrhea), bone marrow suppression.
	Azathioprine	Pancreatitis, dose-related myelosuppression (leukopenia, thrombocytopenia).

⭐ Allopurinol, a xanthine oxidase inhibitor, significantly increases Azathioprine toxicity by blocking its metabolism. Must reduce Azathioprine dose if co-administered.

Sampling Strategy - Peaks vs. Troughs

Trough (C0): Lowest drug level, drawn immediately before the next dose. It assesses for overexposure and toxicity risk.
- Standard for Tacrolimus & Sirolimus.
Peak (C2): Level drawn 2 hours post-dose.
- Sometimes used for Cyclosporine to better estimate total drug exposure (Area Under the Curve - AUC).

⭐ For Calcineurin inhibitors (Tacrolimus, Cyclosporine), trough levels are crucial. Consistently high troughs significantly increase the risk of long-term nephrotoxicity, a key cause of graft failure.

High‑Yield Points - ⚡ Biggest Takeaways

Calcineurin inhibitors (Tacrolimus, Cyclosporine) and mTOR inhibitors have narrow therapeutic indices, demanding close monitoring.

Trough levels (C0) are the standard for monitoring to minimize dose-related toxicities.

Major toxicities include nephrotoxicity for CNIs and myelosuppression for mTOR inhibitors and antimetabolites.

Mycophenolate (MMF) can cause significant leukopenia.

Azathioprine requires TPMT testing to avoid severe myelosuppression.

Watch for drug-drug interactions, particularly with CYP3A4 inhibitors/inducers, which alter drug levels.

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