Cytotoxic immune suppressants and inhibitors of suppression

Question 1

What are cytotoxic immunosuppressants?

Answer 1

1. Act by targeting dividing cells
2. Include alkylating agents, folic acid analogs and purine derivatives
3. Also used as anti-cancer agents
4. Can be very toxic

Question 2

Discuss Alkylators: Cyclophosphamide.

Answer 2

• An inactive form of nitrogen mustard
• Metabolised in the liver to phosphor-amide mustard
• Causes absolute leucopenia with decreased granulocytes, lymphocytes and monocytes; NK cells inhibited

Question 3

Discuss Anti-folates: Methotrexate.

Answer 3

• Act by interfering with dihydrofolate reductase necessary for DNA synthesis
• Inhibits replication of B and T-cells
• Used in rheumatoid arthritis and psoriasis.

Question 4

Discuss dose regulation of methotrexate in cancer and RA.

Answer 4

• Much lower than dose used for cancer
• Anti-inflammatory effect is not inhibited by folic acid supplementation
• MOA is unclear but in RA
  1. MTX is glutamated (long half-life)
  2. Inhibits Aminoimidazole, carboxamidoribonucleotide (AICAR) transformylase
  3. Leads to increased adenosine production
  4. Anti-inflammatory via adenosine receptors

Question 5

Discuss Methotrexate toxicity.

Answer 5

• Folic acid supplementation to prevent side effects.
• Leads to:
  1. Hepatotoxicity
  2. Pulmonary fibrosis/pneumonitis
  3. Myelo-suppression /blood dyscrasias
  4. GI problems (nausea, stomach upset, and loose stools)
  5. Stomatitis or soreness of the mouth
  6. Infection
  7. Alopecia

Question 6

MOA of adenosine.

Answer 6

1. Acts on adenosine receptors:
   - G-protein coupled receptors
   - A1, A2A, A2B, and A3
2. Found on multiple immune cells
3. Acts to increase pro-resolution

Question 7

MOA of Mitoxantrone.

Answer 7

• Anti-cancer agent approved for MS
• Inhibits topoisomerase II
• Intercalates into DNA
• Serious toxicity especially cardio-toxicity
• Last resort therapy

Question 8

MOA of Antibiotics: Doxorubicin

Answer 8

• Anthracycline antibiotics which intercalate with DNA
• Doxorubicin inhibits B-cell function more than T-cell and does not inhibit macrophage or NK function
• Daunorubicin inhibits macrophage function
• Can cause cardiomyopathy

Question 9

MOA of purine analogs: Azathioprine.

Answer 9

• Metabolised to mercaptopurine which inhibits DNA synthesis
• Inhibits antibody production and cell-mediated immunity
• Has no effect on NK cells but inhibits phagocytic cells
• Causes monocytopenia
• Used with prednisone to prevent transplant rejection

Question 10

MOA of Mycophenolate Mofetil.

Answer 10

• Converted to active metabolite mycophenolic acid which inhibits inosine monophosphate dehydrogenase (IMPDH)
• IMPDH is essential for guanine nucleotide synthesis
• Immunosuppressive and anti-proliferative
• Used in renal transplants as prophylaxis with calcineurin inhibitors

Question 11

MOA of Leflunomide.

Answer 11

• Inhibits B- and T-cells
• Mechanism of action is unknown but may involve inhibition of dihydroorotate dehydrogenase (DHODH)
• DHODH is essential for de novo pyrimidine synthesis
• Used for RA
• Side effects are mild GI disturbance
• Active metabolite Teriflunomide also approved for MS

Question 12

MOA of Cladribine.

Answer 12

• Adenosine analogue
• Inhibits adenosine deaminase
• Orally active
• Approved for treatment of hairy cell leukaemia

Question 13

Adverse effects of Cladribine.

Answer 13

• Myelo-suppression
• Neutropenia
• Anemia
• Thrombocytopenia
• Fever
• Infections including sepsis

Question 14

Why was Cladribine development for MS halted?

Answer 14

• Lack of evidence of benefit
• Indication dropped by Merck
• Subsequently approved for MS as a drug of last resort