Immunosuppression/Immunomodulation Flashcards
Why are B and T cells more susceptible to Purine or pyramidine disruption
Because these cells lack an alternate pathway to synthesise these molecules. So without them they cannot synthesis DNA
Which cells are being targeted in treatment of most immune mediated haematological diseases
Th2 - these are overactive resulting in inappropriate stimulation of B cell autoantibody production
Which means it will take up to 7 days to start seeing improvement (half life of IgG)
Treatments also suppress opsonised cell clearance
MOA of corticosteroids
S phase specific
Enters cell via surface receptor (bound to transcortin in blood) which then activates cytoplasmic NFkB preventing its nuclear translocation
The cytoplasmic R then enters the nucleus and associates with glucocorticoid responsive elements (GRE) which include upregulation of anti-inflammatory substances and down regulation of pro-inflammatory substances and functions
NFkB - upregulates expression of multiple inflammatory mediators of the innate system
Net effect is: reduced chemotaxis of inflammatory cells, inhibition of phagocytosis, reduced pro-inflammatory cytokines and chemokines, reduced antigen processing. decrease mast cell number and reduce histamine production. Affects the innate system more than the acquired.
Suppresses T cell function and induces apoptosis, with chronic use there is B cell reduction.
Also antagonises complement.
Other systemic effects of GCS
Healing - inhibit fibroblasts and delay healing
CVS - enhance capillary vasoconstriction, may cause hypertension. Fluid retention increases blood volume.
CNS - lower seizure threshold in some, increased appeitte, alter mood/behaviour
Endocrine - suppression of HPA axis as well as TSH release causing reduction in T4 and reproductive hormones
Insulin resistance
Urinary - reduced ADH response (NDI)
Fluid retention and hypokalaemia
GI - alter mucosal function and risk of ulceration, increase gastric acidity, increase fat absorption decrease iron absorption.
Glycogen and lipid deposits in hepatocytes
Stimulate gluconeogenesis and lipogenesis - specific to abdominal fat
MSK - muscular weakness, atrophy, inhibit bone growth
Skin - thinning, alopecia
GCS Efficacy, metabolism and Adverse effects/contraindications
Efficacy - high in type II hypersensitivity autoimmune disease up to 80% in IMHA, ITP, SRMA. Some evidence that if pred not working and trial dex instead due to slightly different comformation
Dex more potent than pred but has no mineralocorticoid activity. This reduces fluid retention
Variance in dog glucocorticoid receptor may explain the variable severity of side effects in different breeds/large dogs - these may not be the intracytoplasmic receptors but other roles as well.
AEs: PUPD, PP, behaviour change, panting, muscle wastage, ligament rupture, weakness, hypercoagulability, GI upset and ulceration
Alkylating agents used in immunosuppression - MOA
Chlorambucil, cyclophosphamide both non-specific to phase of cell cycle.
Cytotoxic to both resting and dividing immunocompotent cells blocking cell division
Anti-metabolite drugs
Methotrexate,
5-fluorouracil
Cytarabine
Azathioprine
All S phase specific, interfering with DNA synthesis enzymes or causing synthesis of abberrant molecules that fail to function
Azathioprine MOE, metabolism , AEs
Converted to active drugs in liver
Excreted in urine as metabolites
Onset of action can be weeks
Antagonises purine metabolism by causing breaks in DNA and RNA through insertion. Causes T cell apoptosis, inhibits inflammatory gene expression and suppresses APC interactions
-> targets CMI more than humoral because of lack of salvage path for purine synthesis
AEs - contraindicated in cats as more susceptible to myelosuppression (liver deficiency in converting enzymes).
GI upset
Hepatotoxicity (up to 15%)
Changes in hepatic enzymes often resolve with drug discontinuation
Methotrexate MOE, metabolism , AEs
S phase specific anti-metabolite
Binds to folic acid reductase (higher affinity that natural substrate) competitively inhibiting it and affecting purine and pyrimidine production. Thus prevents DNA production and lymphocyte proliferation
Poor oral bioavailability in dogs and high inter-individual variability. Active transport into cells with highest levels seen in kidneys, liver and skin.
AEs: myelosuppression, hepatotoxicity, GI upset, nephrotoxicity at high doses
Vincristine MOE, metabolism , AEs
M phase specific
MOA - binds to cytoplasmic tubulin, toxic to RES macrophages so when ingest platelets containing this they are killed
In addition to inhibiting phagocytosis of platelets, interfering with antiplatelet antibody formation, and preventing the binding of antiplatelet antibodies to platelets, vincristine also accelerates megakaryocytic breakdown and stimulates thrombopoiesis
Extensive hepatic metabolism and excreted in bile
AEs: myelosuppression (neutropaenia exacerbated by cyclosporine), GI upset, neurotoxic, hepatotoxic
Leflunomide MOE, metabolism , AEs
G1 phase arrest
Good oral bioavaliability, converted to active metabolite in stomach
Reversible inhibition of enzyme needed for pyrimidine synthesis -> reduce DNA and RNA synthesis
→ inhibition of autoimmune B and T cell proliferation (as no salvage path for production),
↓ globulin production and impaired leukocyte
Adverse effects appear dose related - usually GI related, hepatotoxicity, myelosuppression, hypercholesterolaemia
Mycophenolate mofetil MOA, metabolism , speed of onset AEs
S phase
Prodrug converted to MPA which inhibits the rate limiting step in denovo synthesis of guanine (depletes purines)
-> selective targeting of B and T lymphocytes with their lack of salvage path.
Suppression of B cell antibody production
once-daily administration is insufficient to maintain therapeutic drug concentrations because MPA has a relatively short half-life
In contrast to the rapid the inhibition of IMPDH by MPA, single and 7-day administration of MMF to healthy dogs did not appear to inhibit lymphocyte proliferation. It therefore remains to be determined whether several weeks of treatment are required to suppress lymphocytes in dog
Limited case controlled studies in vet med but case reports/series indicating efficacy as adjunct in IMPA, IMHA, ITP, SRMA, glomerulonephritis, pemphigus
Side effects: GI upset, ↑ risk of lymphoma, hypertension, peripheral oedema, ↑ risk of infection
NOT myelosuppression or hepatotoxicosis
Cyclosporine A - MOE, metabolism , AEs
Cycle phase G1 - causes arrest
enters T cell cytosol where it binds cyclophilin and calcineurin → blocking signal transduction and transcription factor NF-AT resulting in reduced production of IL-2 and IFN-y by T cells (predominantly Th1 thus inhibits CMI response)
Indirect suppressive effects on B cells, NK cells, neuts, mast cells through reduction in GM-CSF, TNFa IL 3 and 4
Substrate for PGp efflux so different formulations are not equivocal in bioavailability - in cats this is more variable than dogs.
Vet product has good bioavailability in fasted state, it is protein bound in circulation and distributes in to liver, fat and blood cells with significant accumulation in skin (lower doses for skin dz)
Primarily undergoes liver metabolism (affected by other drugs using cyp450 system) and eliminated in the bile.
Food ↓ oral absorption in dogs, variable bioavailability
Side effects; opportunistic infections, emergence of neoplasia. GIT (usually improves without withdrawal), risk of DM Renal disease in people. Gingival hyperplasia. Hypertrichosis. Rare idiosyncratic hepatotoxicity, fatal systemic toxoplasmosis in cats, acute bullous keratopathy from latent FeHV flares
In a study specifically addressing adverse effects in cats treated with CsA, vomiting occurred in 12% of cats and soft stools or diarrhoea in 16% of cats. Anorexia in 2-10% and weight loss in 5-16% of treated cats. Gingival hyperplasia due to CsA effect on fibroblasts has isolated reports
Transplanted cats had more than six times higher odds of developing malignant neoplasia compared with control cats, but the specific role of CsA remains undetermined
In cats treated with CsA at the recommended dose for allergic or immune-mediated diseases, malignant neoplasia appears to be a rare occurrence
Cyclosporine monitoring - recommendations and controversy
Drug levels - need to be whole blood as drug accumulates in cells, 2h post administration after 2 weeks of therapy.
Therapeutic range for dogs is reported as 400-600ng/ml but non-responders have been seen at higher levels, as well as good responses at lower levels.
In cats no correlation of blood levels with clinical response has been found in the limited studies
Seems to be variable sensitivity of patient calcineurin to cyclosporine –>
Recent development of downstream monitoring of efficacy through measurement of activated T cell IL 2 and IFNy expression via qRTPCR.
Assessed in first weeks of cSA therapy to establish efficacy and subsequent weeks if patient fails to demonstrate clinical response.
If excessive immunosuppression is suspected due to unexpected infections.
Only need peak for BID dosing, but peak and trough for SID.
Also need to monitor chemistrys every 3-6 months
Interpretation of cyclosporine pharmacodynamic results
Marked suppression - Tcell inability of IL2 production
-> reduce dose if well controlled, increased dose unlikely to be beneficial if not controlled.
Moderate/High - still some t cell ability to make IL2, not maximally suppressed. Only small dose increase would be recommended if not well controlled.
Moderate - cells retain reasonable ability to make IL2. Reasonable to give dose increase if not adequately controlled
Low - if well controlled no need to adjust but if not well controlled consider dose increase if availability issues have been ruled out.
