Immunosuppressive Agents

Comprehensive question-and-answer flashcards covering immunosuppressive pharmacology, including glucocorticoids, cytotoxic alkylating agents and antimetabolites, calcineurin inhibitors, mTOR inhibitors, monoclonal antibodies, and transplantation regimens.

According to the lecture notes, what are the four conditions or entities that the immune response acts as a sophisticated defense system against?

1. Infectious micro-organisms
2. Foreign (transplant) cells
3. Transformed (cancer) cells
4. Autologous cells (autoimmunity)

What specific immune effects do the cytokines IL-2, IL-4, IFN-γ, and TNF-α mediate according to the cellular differentiation summary?

• IL-2: Activation and proliferation of cytotoxic T-cells • IL-4: Activation of T- and B-cells and macrophages • IFN-γ: Activation of T-cells and macrophages, increased MHC expression • TNF-α: Stimulates B-cell proliferation, cytotoxic effect on tumor cells

What are the four recognized categories of graft rejection severity and the two stages of graft rejection?

Severity categories:

1. Hyper-acute rejection
2. Accelerated rejection
3. Acute rejection
4. Chronic rejection

Stages:

1. Afferent stage
2. Efferent stage

According to the 2022 US transplantation data from UNOS.org, how many total organ transplants were performed, and how many of those were kidneys and livers?

A record total of $42{,}887$ organ transplants were performed in 2022, which included $25{,}498$ kidneys and $9{,}528$ livers ($4{,}111$ hearts and $2{,}692$ lungs were also performed, with $103{,}770$ people on the waitlist as of June 12, 2023).

How do glucocorticoids exert their immunosuppressive effects at the molecular level regarding NF-κB and cytokine production?

Glucocorticoids bind intracellular glucocorticoid receptors and inhibit transcription factor NF-κB via transrepression (preventing binding to the NF-κB responsive element). This down-regulates the synthesis of pro-inflammatory cytokines including IL-1, IL-2, IL-6, IFN-γ, and TNF-α.

Why is abrupt cessation of prolonged glucocorticoid therapy contraindicated, and how long can full recovery of the HPA-axis take?

Abrupt cessation must be avoided due to suppression of the hypothalamic-pituitary-adrenal (HPA) axis; full recovery of the HPA-axis can take up to $1$ year.

What are the key clinical manifestations of Cushing's syndrome caused by excess glucocorticoid exposure?

• Centripetal adipose redistribution, moon face, and buffalo hump • Proximal limb myopathy and thin arms/legs • Osteoporosis (due to decreased GIT Ca2+ absorption and decreased renal reabsorption) • Immunosuppression (increased vulnerability to infection) • Diabetes mellitus and hypertension/salt retention • Hirsutism, poor wound healing, peptic ulcers, and growth retardation in children

What cytotoxic and toxic metabolites are generated during the bioactivation of cyclophosphamide by hepatic CYP450?

Cyclophosphamide is converted by CYP450 to 4-hydroxycyclophosphamide, which equilibrates with aldophosphamide. Aldophosphamide is broken down non-enzymatically into cytotoxic phosphoramide mustard and toxic acrolein.

What is the molecular mechanism of action of cyclophosphamide's active metabolite, phosphoramide mustard?

It acts as a bifunctional alkylating agent, covalently crosslinking adjacent strands of DNA primarily at the $N\text{-}7$ position of deoxyguanylates. This causes abnormal base pairing, strand breakage, prevents strand separation during replication (most susceptible in late $G_1$ and $S$ phase), and expresses a block in $G_2$.

What serious bladder toxicity is associated with cyclophosphamide therapy, what causes it, and how is it prevented?

Hemorrhagic cystitis, caused by the toxic metabolite acrolein (which also leads to a $9\text{ to }45\times$ increased risk of bladder cancer). It is prevented by vigorous hydration and administration of sodium 2-mercaptoethane sulfonate (MESNA).

What is the mechanism of action of methotrexate (MTX), and which cell cycle phase is it specific for?

Methotrexate is a structural analog of dihydrofolic acid ($\text{FH}_2$) that reversibly inhibits dihydrofolate reductase (DHFR/DHR), inhibiting purine and pyrimidine synthesis. It is specific for the $S$ phase of the cell cycle.

How is azathioprine (AZA) converted into its active cytotoxic metabolites, and what is its primary immunological target?

Azathioprine is a prodrug metabolized to 6-mercaptopurine (6-MP), which is converted by HGPRT to thioinosinic acid (TIMP), leading to 6-methylmercaptopurine nucleotide (6-MeMPN, which inhibits de novo purine synthesis) and 6-thioguanine nucleotides (6-TGN, which incorporate into DNA). It primarily targets cell-mediated immune responses.

What dangerous drug-drug interaction occurs when azathioprine is co-administered with allopurinol, and what adjustment is required?

Allopurinol blocks xanthine oxidase (XO), inhibiting 6-MP breakdown and increasing 6-MP bioavailability by $4\times$, resulting in life-threatening pancytopenia. The azathioprine dose must be reduced by $\frac{1}{4}$ (to $25\%$ of normal) or the patient should be switched to mycophenolate mofetil (MMF).

What is the mechanism of action of leflunomide, and what agent is used to treat an overdose?

Leflunomide is metabolized to active teriflunomide, which inhibits dihydroorotate dehydrogenase, decreasing pyrimidine synthesis and lymphocyte activation (most effective against B-cells). In overdose, cholestyramine is administered to interrupt enterohepatic circulation.

What specific enzyme is inhibited by mycophenolate mofetil (MMF), and why are lymphocytes preferentially affected?

Mycophenolate mofetil inhibits inosine monophosphate dehydrogenase (IMPDH), specifically isoform II. Lymphocytes are preferentially affected because they rely exclusively on the de novo pathway for purine synthesis, whereas other cell types can utilize the salvage pathway.

In the immunosuppressive signaling pathway shown, how do Cyclosporine and Tacrolimus inhibit IL-2 gene transcription?

Cyclosporine binds to Cyclophilin, while Tacrolimus binds to FKBP. Each drug-protein complex binds and inactivates calcineurin, preventing the dephosphorylation of inactive NFAT-P to active NFAT. As a result, NFAT cannot translocate to the nucleus to induce transcription of the IL-2 gene.

What are the common adverse effects of cyclosporine A (CsA), and how does its bone marrow toxicity compare to other cytotoxic immunosuppressants?

CsA causes nephrotoxicity (in $\sim 75\%$ of patients), hypertension (in $50\%$), neurotoxicity/tremors (in $20\%$), hirsutism, gingival hyperplasia, hepatotoxicity with cholestasis, and post-transplant hyperglycemia. Unlike cytotoxic antimetabolites, CsA exhibits minimal bone marrow toxicity.

How does the side effect profile of tacrolimus (FK506) differ from that of cyclosporine A regarding hair changes, diabetes, and gastrointestinal effects?

• Hair changes: Tacrolimus causes alopecia, whereas cyclosporine causes hirsutism. • Diabetes: Tacrolimus has a higher incidence of post-transplant Type 1 diabetes ($20\%$ of patients, especially in Black and Hispanic populations). • Gastrointestinal effects: Tacrolimus causes a higher incidence of diarrhea and nausea than CsA.

What is the mechanism of action of sirolimus (rapamycin), and how does its molecular target differ from calcineurin inhibitors?

Sirolimus binds to FKBP, and this complex inhibits the mechanistic target of rapamycin (mTOR). Instead of blocking calcineurin-mediated IL-2 gene transcription, sirolimus inhibits downstream IL-2 receptor signaling (inhibiting p70 S6 kinase and PHAS-1), arresting cell cycle progression in T and B lymphocytes.

What unique clinical application does sirolimus have in interventional cardiology, and what are its characteristic adverse effects?

Sirolimus is used in sirolimus-coated coronary stents to inhibit restenosis by reducing endothelial cell proliferation. Its characteristic adverse effects include myelosuppression (especially thrombocytopenia), hypertriglyceridemia, and hepatotoxicity, with notably minimal nephrotoxicity.

What is the mechanism of action of polyclonal antithymocyte globulin (ATG) and antilymphocyte globulin (ALG), and what is a characteristic immune-mediated toxicity?

They bind to the surface of circulating T-lymphocytes, leading to complement-mediated lysis and phagocytosis by the liver and spleen (causing profound lymphopenia). A characteristic toxicity is serum sickness and nephritis resulting from host immune responses against foreign proteins.

What is basiliximab, what is its approved clinical indication, and when is it administered?

Basiliximab is an anti-IL-2 receptor (CD25) monoclonal antibody approved for induction therapy in renal transplantation. It is administered intravenously as a two-dose regimen: $20\text{ to }30\text{ min}$ preoperatively and on the $4\text{th}$ postoperative day.

How do the three TNF-α inhibitors infliximab, etanercept, and adalimumab differ in their structural design?

• Infliximab: A human-mouse chimeric IgG1 monoclonal antibody targeting TNF-α • Etanercept: A recombinant fusion protein consisting of the TNF receptor linked to IgG Fc that binds both TNF-α and TNF-β • Adalimumab: A fully human monoclonal antibody directed against TNF-α

What is belatacept, what is its indication, and what is its specific intravenous dosing schedule?

Belatacept is an immunomodulatory fusion protein that blocks T-cell costimulation/activation, indicated solely for prophylaxis of organ rejection in renal transplants. Its IV dosing schedule is: Day 0, Day 5, Weeks 2, 4, 8, 12, and every month thereafter.

What specific drugs comprise the standard triple-drug maintenance regimen used early post-transplant in cadaveric organ recipients?

1. A calcineurin inhibitor (Cyclosporine A or Tacrolimus)
2. A cytotoxic antiproliferative agent (Azathioprine or Mycophenolate mofetil)
3. A corticosteroid (e.g., Prednisone)

What three major long-term complications limit the success of chronic immunosuppressive therapy in organ transplant recipients?

1. Transplant-associated vasculopathy (a progressive form of occlusive coronary artery disease)
2. Opportunistic and common infections (due to non-specific immune suppression)
3. Malignancy (due to downgrading of immune surveillance allowing proliferation of oncogenic viruses and spontaneously arising malignant cells, as seen in Isabelle Dinoire)