Prediction of metabolic clearance: CYP2C9 genotyping in epileptic patients on phenytoin

1. Prediction of metabolic clearance: CYP2C9 genotyping in epileptic patients on phenytoin

2. Epilepsy prevalence: 1 / 150 (2nd only to migraine) excessive neuron stimulation within CNS brain messages become halted / confused seizure: burst of electrical activity in brain generalised or partial Group of disorders characterized Focal: simple or complex Genralised: Convulsive (tonic-clonic)/Grand Mal Nonconvulsive (absence)/Petite MalGroup of disorders characterized Focal: simple or complex Genralised: Convulsive (tonic-clonic)/Grand Mal Nonconvulsive (absence)/Petite Mal

4. The Neuron axon - the long extension of a neuron that carries nerve impulses away from the body of the cell. axon terminals - the hair-like ends of the axon cell body - the cell body of the neuron; it contains the nucleus (also called the soma) dendrites - the branching structure of a neuron that receives messages (attached to the cell body) myelin sheath - the fatty substance that surrounds and protects some nerve fibers node of Ranvier - one of the many gaps in the myelin sheath - this is where the action potential occurs during saltatory conduction along the axon nucleus - the organelle in the cell body of the neuron that contains the genetic material of the cell Schwann's cells - cells that produce myelin - they are located within the myelin sheath. Green – axon Red - dendriteaxon - the long extension of a neuron that carries nerve impulses away from the body of the cell. axon terminals - the hair-like ends of the axon cell body - the cell body of the neuron; it contains the nucleus (also called the soma) dendrites - the branching structure of a neuron that receives messages (attached to the cell body) myelin sheath - the fatty substance that surrounds and protects some nerve fibers node of Ranvier - one of the many gaps in the myelin sheath - this is where the action potential occurs during saltatory conduction along the axon nucleus - the organelle in the cell body of the neuron that contains the genetic material of the cell Schwann's cells - cells that produce myelin - they are located within the myelin sheath. Green – axon Red - dendrite

5. Anti-epileptic drugs at the synapse

6. PHT clearance kinetics A 10% increase in dose may result in a 2-3 x C increase A 10% increase in dose may result in a 2-3 x C increase

7. Dose / [PHT] relationship

8. Factors affecting phenytoin levels liver disease GI disease renal function age Changes in hepatic blood flow do not alter phenytoin clearance because it is a low-extraction drug.10 However, loss of functional hepatocytes decreases phenytoin metabolism. Phenytoin accumulates as hepatic dysfunction increases.11 Because of decreased albumin production, liver disease is associated with decreased protein binding capacity for phenytoin. Further, bilirubin may compete for binding sites of the albumin molecule, further increasing the unbound concentration.12-17 The absorption of phenytoin is pH dependent and is maximal in the duodenum.9 Diseases that alter gut pH and decrease duodenal dwell time are likely to alter the absorption of phenytoin. Phenytoin binding to plasma protein is decreased in patients with renal failure and hypoalbuminemia Age-drugs common together too Drugs: Chloramphenicol increases serum concentrations by 20% Barbiturates acutely decrease phenytoin clearance (competitive), long-term enzyme induction increases clearance Changes in hepatic blood flow do not alter phenytoin clearance because it is a low-extraction drug.10 However, loss of functional hepatocytes decreases phenytoin metabolism. Phenytoin accumulates as hepatic dysfunction increases.11 Because of decreased albumin production, liver disease is associated with decreased protein binding capacity for phenytoin. Further, bilirubin may compete for binding sites of the albumin molecule, further increasing the unbound concentration.12-17 The absorption of phenytoin is pH dependent and is maximal in the duodenum.9 Diseases that alter gut pH and decrease duodenal dwell time are likely to alter the absorption of phenytoin. Phenytoin binding to plasma protein is decreased in patients with renal failure and hypoalbuminemia Age-drugs common together too Drugs: Chloramphenicol increases serum concentrations by 20% Barbiturates acutely decrease phenytoin clearance (competitive), long-term enzyme induction increases clearance

10. CYP2C9 clinical relevance metabolises 20 % of all drugs warfarin, phenytoin, tolbutamide, ibuprofen polymorphism point mutations *1 (wild type), *2 & *3 (poor metabolisers) Anti-coagulant, anti-convulsant, diabetes control, anti-inflammatoryAnti-coagulant, anti-convulsant, diabetes control, anti-inflammatory

11. Hypothesis ‘ ability to clear Phenytoin is a function of CYP2C9 genotype ’

12. PCR-RFLP

13. Dose-Response

14. Determination of Km and Vmax

15. Pharmacokinetics

16. Summary

17. Conclusions CYP2C9 ‘poor metabolisers’ can be phenotypically categorised: lower Vmax higher Km maintained on lower PHT doses PHT dose prediction should consider: CYP2C9 genotype (CYP2C19) Current medication

18. Acknowledgements Walton Centre for Neurology & Neurosurgery (WCNN) Neurobiochemistry lab Secretarial staff Dr. David Britt (research governance) Dr. Tony Marson, Dr. Siva Sathasivum, Prof. David Chadwick (epilepsy clinic) University of Liverpool Dr. Ana Alfirivic, Prof. Munir Pirmohamed (Pharmacology) Jenny Austin, Dr. Lazlo Pazmany (Rheumatology, Clinical Sciences UHA) University Hospital Aintree Dr. Ian Watson