Modifiers of Metabolism New Frontier of Antidotal Therapy

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Modifiers ofMetabolismNew Frontier
ofAntidotal Therapy
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CarnitineAn Antidote(?) forMitochondrial Toxins

• Jeffrey Suchard, MD FACEP FACMT
• Associate Professor of Clinical Emergency
  Medicine
• Director of Medical Toxicology
• Department of Emergency Medicine
• University of California, Irvine Medical Center
• e-mail jsuchard_at_uci.edu

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Goals / Objectives

• Discuss the use of Carnitine to treat toxicity
  from agents that inhibit mitochondrial fatty acid
  ?-oxidation
• particularly Valproic acid
• Review the ongoing research regarding carnitines
  role as an antidote
• Describe potential future areas for carnitine
  toxicologic research

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The Big Picture What the world looks like to the
mitochondria in your liver

• Each cycle of ß-oxidation removes a 2-carbon unit
  (acetyl-CoA) from fatty acid
• NADH produced powers the electron transport chain
• Acetyl-CoA enters Krebs cycle, producing more
  NADH, FADH2, and GTP
• Lots of ATP per fatty acid!

• Main job produce Acetyl Coenzyme A
• the unit of currency for cellular energy
• We will focus on fatty acid oxidation
• How to turn long carbon chains (fatty acids) into
  two-carbon units (acetyl-CoA)

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Fatty Acid ß-Oxidation

• Occurs in the mitochondrial matrix
• But how do the fatty acids get there in the first
  place?
• Carboxylic acids are charged, and dont cross
  membranes easily
• There are two lipid bilayer membranes to cross in
  order to enter the matrix

• Carnitine Shuttle
• Carnitine essential carrier molecule for fatty
  acid entry into mitochondrial matrix

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Causes of ImpairedFatty Acid ß-Oxidation

• Genetic
• ß-Oxidation enzyme deficiencies
• MCAD deficiency is most common
• Carnitine and/or CoA sequestration

• Exogenous
• Reyes Syndrome
• Viral illness impairsß-Oxidation
• So does aspirin!
• Dideoxynucleosides
• Used for HIV
• Impairs mitochondrial DNA synthesis
• Ackee fruit
• Jamaican vomiting sickness
• Valproic acid

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Consequences of ImpairedFatty Acid ß-Oxidation

• Microvesicular Steatosis
• Form of fatty liver from accumulation of
  non-esterified fatty acids (NEFA)

• ? ATP from fat, yet gluconeogenesis also impaired
• Hypoglycemia
• Other pathways impaired by accumulated abnormal
  metabolites
• Krebs cycle
• Urea cycle
• Oxidative phosphorylation
• Electron transport

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Toxidrome of Impairedß-Oxidation

• Abnormal Labs
• Metabolic Acidosis
• Lactic Acidosis
• Hypoglycemia
• Hypoketonemia
• Hyperammonemia
• Dicarboxylic Acidemia
• FA undergo ?-oxidation
• Hepatic Injury
• ? LFTs, ? PT

• Abnormal Cell Structure
• Microvesicular Steatosis
• Accumulated NEFAs
• Abnormal Mitochondria

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Valproic Acid

• The most commonly encountered agent that
  significantly inhibits fatty acidß-oxidation
• Therapeutic use
• Overdose
• One of the most studied exogenous inhibitors of
  fatty acid ß-oxidation

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Valproic Acid

• Short-chain, branchedfatty acid
• 2-n-propylpentanoic acid
• dipropylacetic acid
• Initially handled likeother fatty acids
• Transformed into valproyl-CoAand
  valproyl-carnitine
• sequesters carnitine intra-mitochondrial CoA
• can lead to inhibition of ?-oxidation of FAs

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Valproate (VPA) Metabolism

• Sequesters carnitine
• ? renal loss
• Indirectly inhibits normal FA ß-oxidation
• VPA undergoesß-oxidation and?-oxidation,
  forming multiple metabolites
• 4-en-VPA seems tobe the most toxic

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Mitochondrial Toxicity in Therapeutic VPA Dosing

• Asymptomatic ? LFTs and ? NH3 common
• Up to 44 of patients in first 3 months
• ? VPA hepatotoxicity with concurrent CYP-inducing
  anticonvulsants
• Mediated via ? 4-en-VPA formation
• Infrequent Reyes-like syndrome
• Mainly in children, 1st to 4th month of therapy

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VPA HepatotoxicityHyperammonemia

• Defined as NH3 level gt 80 ?g/dL
• Occurs in 35-45 of patients
• Proposed mechanism is
• Urea Cycle Dysfunction
• How does VPA / inhibition of ?-oxidation affect
  the urea cycle?

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Speculative Effects of VPA on Intermediary
Metabolism

• VPA requires Carnitine to enter mitochondria
• Valproylcarnitine ester eliminated in urine
• Results in relative carnitine deficiency
• VPA also inhibits ?-oxidation by sequestering
  unbound CoASH
• Limits formation of Fatty Acyl-CoAs
• Limits formation of Acetyl-CoA
• ? activation of pyruvate carboxylase
• ? levels of Aspartate and Glutamate

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L-Carnitine

• 3-hydroxy-4-trimethylammonium butyrate
• Modulates Fatty Acyl-CoA/CoASH ratios
• Acyl-CoA Carnitine ? Acylcarnitine CoASH
• Supplementation converts Valproyl-CoA to
  Valproylcarnitine ? excreted in the urine
• Frees up Coenzyme A, and restores normal Fatty
  Acyl-CoA/CoASH ratios
• Allows ?-oxidation to occur unimpeded

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Pharmaceutical Carnitine

• Only commercially-available drug is Carnitor
  (Sigma-Tau Pharmaceuticals)
• FDA-approved for
• Primary systemic carnitine deficiency
• Inborn errors of metabolism resultingin
  secondary carnitine deficiency
• Prevention/treatment of carnitinedeficiency in
  ESRD patientsundergoing dialysis
• Not approved for VPAor other ß-ox inhibitors!

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Carnitine Side-Effects

• Per product insert
• Nausea / Vomiting
• Gastritis
• Seizures
• Body Odor
• Smells like rotting fish!
• Trimethylamine(?)

• When used as antidotal therapyfor VPA toxicity
• 251 doses of carnitine given to VPA overdose
  patients with ?NH3 with no allergic rxns or
  side-effects reported
• LoVecchio et al.Am J Emerg Med 200523321

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Who Needs Carnitine?

• A subject of potential antidotal research
• Consensus statement Epilepsia 1998391216
• Carnitine is clearly indicated in cases of
  VPA-induced hepatoxicity, VPA overdose, 1?
  carnitine-transporter defect
• Strongly recommended also for
• 2? carnitine deficiency syndromes, symptomatic
  VPA-associated hyperammonemia, renal symptoms for
  dialysis patients, kids lt2 yrs on multiple
  anticonvulsants, seizure patients on ketogenic
  diets, premies on TPN

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Evidence for Carnitine Efficacy

• Mostly from Pediatric Neurology literature
• All 15 pts (35 of study population)with NH3
  gt100 had significant decreases in NH3 when given
  supplemental carnitine
• Bohles et al. Acta Paediatr 199685446.

• A child with VPA overdose (serum level 1316)
  had ? ?-oxidation products (inc. 4-en-VPA) and ?
  ß-oxidation products, which improved after IV and
  PO carnitine supplementation
• Ishikura et al. J Anal Toxicol 19962055

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Evidence for Carnitine Efficacy

• Among 92 pts with severe VPA-induced
  hepatotoxicity in a retrospective review,50
  survived when given carnitine versus only 10
  with supportive care
• Bohan et al. Neurology 2001561405

• Serum NH3 half-life decreased from42.1 ( 42.1)
  hrs to6.1 ( 3.6) hrs
• Statistically non-significant
• Various tx modalities, inc. hemodialysis
• Sztajnkrycer et al. J Toxicol Clin Toxicol
  200139497

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Summary ofEvidence for Carnitine Efficacy

• There are no blinded or prospective studies
  showing improved outcome with carnitine therapy
• e.g. decreased length-of-stay
• However, some metabolic measures of toxicity
  appear to improve with carnitine
• Serum ammonia decreases
• ?-oxidation products decrease

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What are the current indications for using
Carnitine in VPA overdose?

• IV carnitine is recommended for
• Coma
• Climbing serum NH3 levels
• Patients with VPA level gt450 mg/L
• Perez A, McKay CA. Role of carnitine in valproic
  acid toxicity.J Toxicol Clin Toxicol 200341899
• Some advocate that hyperammonemia is the 1?
  (sole?) indication
• Sztajnkrycer. J Toxicol Clin Toxicol 200240789.

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Carnitine Dosing for VPA Toxicity

• Optimal dose remains undetermined
• More potential research questions
• Is there a minimum effective dose?
• Does this really matter?
• Why not give a lot, since carnitine is fairly
  safe?
• Does route of dosing matter?
• IV vs. PO

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Carnitine Dosing Regimens

• 1996 Pediatric Neurology Advisory Committee
  consensus guidelines
• 100 mg/kg/day (? 2g) PO in divided doses
• Kids on chronic VPA tx
• Manufacturer recs
• Adult 990 mg PO 2-3/day
• Peds 50-100 mg/kg/day PO in divided doses
  (? 3g)

• Reported regimens for Overdose / ?NH3
• 150-500 mg/kg/day (? 3g) intravenously
• 100 mg/kg IV load,then 250 mg/kg q8h
• 150 mg/kg IV load,then 500 mg q8h
• 25 mg/kg IV q6h
• 100 mg/kg/day NG
• 3g PO load, then660 mg PO q8h

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Most Recent Recommendations

• Acute Overdose without Hepatotoxicity
• 100 mg/kg/day, divided q6h
• Up to 3g/day
• Didnt specify whether carnitine was to be given
  PO or IV, but probably PO

• Acute Overdose with symptomatic Hyperammonemiaor
  Hepatotoxicity
• 100 mg/kg IV load over 30 min (max. 6g)
• then 15 mg/kg q4h over 10-30 min until clinical
  improvement occurs

Russell S. Carnitine as an antidote for acute
valproate toxicity in children. Curr Opin
Pediatr 200719206 Review article