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Propofol vs Fospropofol - Comprehensive MD Final Exam Review

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1. Chemistry and Formulation

Propofol

• Class: Alkylphenol derivative - 2,6-diisopropylphenol
• Structural formula:

• Highly lipid-soluble, insoluble in aqueous solution
• Standard formulation: 1% propofol + 10% soybean oil + 1.2% purified egg phospholipid (emulsifier) + 2.25% glycerol (tonicity) + NaOH (pH 7)
• EDTA added for bacteriostatic activity (prevents microbial growth in high-fat emulsion)
• Appearance: slightly viscous, milky white substance (small lipid droplets)
• Available in 1% (standard) and 2% formulations; European formulations also use medium-chain + long-chain triglycerides
• Stable at room temperature; not light-sensitive; may be diluted with 5% dextrose in water

Fospropofol (Lusedra)

• Class: Water-soluble prodrug of propofol; disodium phosphonooxymethyl ester of propofol
• FDA approved December 2008 for monitored anesthesia care (MAC) in adults
• Formulation: sterile, aqueous, colorless, clear solution; 35 mg/mL; single-dose vial
• 1.86 mg fospropofol = molar equivalent of 1 mg propofol
• Metabolized by alkaline phosphatases in the liver → propofol + phosphate + formaldehyde
  • Formaldehyde further metabolized by aldehyde dehydrogenase (liver and erythrocytes)
• Classified as a Schedule V controlled substance by the DEA (2009)

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2. Mechanism of Action

• Primary: Positive allosteric modulator of GABA-A receptors - enhances Cl⁻ conductance, potentiates GABA inhibitory neurotransmission
• Also: Inhibits NMDA receptors, modulates slow calcium ion channels
• Anticonvulsant properties via GABA agonism and NMDA inhibition
• At high concentrations: antipruritic effects (unique to propofol among anesthetics)

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3. Pharmacokinetics

Propofol PK

• Elderly (>65 yrs, esp. >80 yrs): 50% lower dose than 20-year-olds; decreased clearance + increased CNS sensitivity
• Children: Larger central compartment (50% more) + faster clearance (25% faster); children <3 yrs: even larger Vd and higher clearance → larger dose requirements
• Women: Larger Vd + higher clearance, but similar elimination t½ to men
• Hepatic disease: Larger steady-state and central compartment volumes; clearance unchanged; slight t½ prolongation - no significant dose adjustment needed (extrahepatic clearance compensates)
• Renal disease: Pharmacokinetics unaltered
• Hemorrhagic shock: Blood concentrations increase; dose requirements decrease 38-54% due to slower intercompartmental clearances and leftward shift of concentration-effect relationship

• Midazolam (200 ng/mL) raises propofol concentrations ~25% by reducing metabolic clearance
• Alfentanil: Increases propofol concentrations by reducing elimination and distribution clearances
• Propofol increases remifentanil concentrations (↓ central Vd and distributional clearance by 41%, elimination clearance by 15%)
• Epidural ropivacaine (20-segment block): Reduces propofol dose/target by ~30%

Fospropofol PK

• Multicompartment model: 2-compartment for fospropofol + 3-compartment for liberated propofol
• After 6 mg/kg IV bolus: Parent drug peaks at 4 minutes; liberated propofol peaks at 12 minutes (delayed onset)
• Terminal elimination t½ of fospropofol: 0.88 hours
• Not affected by: Race, sex, mild-to-moderate renal impairment, age, or alkaline phosphatase activity levels
• Recovery is prolonged compared to propofol (prodrug must be converted first)
• Note: 6 studies on PK/PD published pre-2010 were retracted due to analytical assay inaccuracy - published data have significant limitations

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4. Pharmacodynamics / Organ System Effects

Central Nervous System

Cardiovascular System

• 25-40% decrease in systolic BP after induction dose (2-2.5 mg/kg)
• Mechanism of hypotension: combined vasodilation + ↓ cardiac output + ↓ cardiac index (±15%), ↓ stroke volume index (±20%), ↓ SVR (15-25%), ↓ LVSWI (±30%)
• Vasodilation mechanisms: direct inhibition of smooth muscle calcium mobilization, inhibition of prostacyclin synthesis, reduction of angiotensin II-mediated calcium entry, K-ATP channel activation, ↑ nitric oxide
• Baroreflex impairment: Propofol resets/inhibits baroreflex → blunts tachycardic response to hypotension
• Heart rate: usually unchanged (↓ parasympathetic tone, but baroreflex impaired)
• Rare: severe bradycardia via Bezold-Jarisch reflex (vagally mediated from marked drop in cardiac filling)
• Hemodynamic depression lags behind hypnotic effect: t½ effect-site = 2-3 min for hypnosis vs. ~7 min for hemodynamic depression
• In hemorrhagic shock: hemodynamic depression markedly potentiated
• Cardioprotection: Postoperative troponin levels lower with propofol vs. sevoflurane in cardiac surgery (two large studies)

Respiratory System

• Profound respiratory depressant
• Apnea after induction dose
• Even at subanesthetic doses: inhibits hypoxic ventilatory drive, depresses response to hypercarbia
• Upper airway reflex depression exceeds thiopental → allows intubation/LMA/endoscopy without NMB
• Less bronchospasm than barbiturates or etomidate; lower histamine release → preferred in asthmatics
• KEY WARNING: Only trained airway management personnel should administer propofol for sedation

Neuromuscular System

• No effect on evoked EMG or twitch tension
• Acceptable intubating conditions after propofol alone reported

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5. Dosing

Propofol Dosing

Fospropofol Dosing (MAC sedation)

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6. Adverse Effects - High-Yield Exam Points

Propofol Adverse Effects

Propofol Infusion Syndrome (PRIS) - Must Know

• Definition: Rare, potentially fatal syndrome
• Threshold: Infusion ≥4 mg/kg/h for ≥48 hours (cases at lower doses/shorter duration reported)
• First described in: Children; later observed in critically ill adults
• Cardinal feature: Acute refractory bradycardia → asystole + ONE of:
  • Metabolic acidosis (base deficit >10 mmol/L)
  • Rhabdomyolysis
  • Hyperlipidemia
  • Enlarged/fatty liver
• Other features: Cardiomyopathy, acute cardiac failure, skeletal myopathy, hyperkalemia, hepatomegaly, lipemia
• Major risk factors: Poor O₂ delivery, sepsis, serious cerebral injury, large propofol dose
• Predisposing factors: Genetic disorders of fatty acid metabolism (MCAD deficiency), low carbohydrate supply
• Warning sign: Rising lipemia = early indicator of impending PRIS
• ACCA/CCM guidelines: Monitor for unexplained metabolic acidosis or arrhythmias in ICU patients on propofol

Fospropofol Adverse Effects

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7. Clinical Uses and Indications

Propofol

• Induction and maintenance of general anesthesia (most common IV induction agent in USA)
• TIVA (Total Intravenous Anesthesia) - often combined with remifentanil, dexmedetomidine, or ketamine
• ICU sedation - mechanically ventilated patients (long-term feasible due to PK profile; weigh against PRIS risk)
• MAC sedation for procedures
• PONV treatment/prophylaxis - subhypnotic doses (10-20 mg)
• Status epilepticus - treatment
• Antipruritic - e.g., cholestatic pruritus
• Target-Controlled Infusion (TCI) - used extensively outside USA; uses PK models (Marsh, Schnider) based on patient characteristics
• Preferred in outpatient anesthesia due to PONV reduction and rapid clear-headed recovery
• Safe in malignant hyperthermia-susceptible patients

Fospropofol

• Approved use: MAC sedation during diagnostic and therapeutic procedures in adults ONLY
• Studies showed superiority to midazolam + fentanyl for endoscopy: more complete amnesia + better sedation
• NOT established for: pediatric use, ICU sedation, general anesthesia induction, TIVA
• Its place relative to other agents remains uncertain in clinical practice (Morgan & Mikhail)

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8. Side-by-Side Comparison Table

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9. Contraindications and Cautions

Propofol

• Allergy: Egg/soy allergy (standard emulsion) - caution; though the phospholipid is highly refined, cross-reactivity is debated
• Pediatric ICU sedation for prolonged periods: FDA recommends against it
• Elevated ICP: Use with extreme care (can critically reduce CPP); limit to mild-moderate sedation (blood concentration ~2 mcg/mL)
• Hemodynamic instability: High risk of severe hypotension
• Hypertriglyceridemia/pancreatitis history: Caution with lipid load

Fospropofol

• Not approved for pediatric use
• Formaldehyde metabolite - theoretical concern in patients with aldehyde dehydrogenase deficiency
• ASA III/IV and elderly: 25% dose reduction required
• Weight-capping at 60-90 kg (unique to fospropofol dosing)

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10. Key Points for MD Final Exam

1. Propofol causes dose-dependent hypotension - mechanism is combined vasodilation (via NO, Ca²⁺ channel inhibition, K-ATP activation) + cardiac depression; baroreflex is impaired
2. Hemodynamic depression lags hypnosis by ~5 minutes (effect-site t½: 2-3 min hypnosis vs. ~7 min hemodynamic)
3. Fospropofol = prodrug metabolized by alkaline phosphatase → propofol + phosphate + formaldehyde
4. 1.86 mg fospropofol = 1 mg propofol (molar equivalence)
5. 74% incidence of perineal paresthesia with fospropofol - its most distinctive adverse effect
6. PRIS threshold: ≥4 mg/kg/h for ≥48h; key triad = bradycardia + metabolic acidosis + rhabdomyolysis
7. Fospropofol is Schedule V (DEA) - propofol is not scheduled
8. Propofol safe in MH - does not trigger malignant hyperthermia
9. Propofol useful in asthmatics - less bronchospasm and histamine release vs. barbiturates
10. Propofol decreases ICP by 30-50% but also decreases CPP - critical in head injury management
11. Elderly patients (>80 yrs): 50% dose reduction of propofol needed
12. Propofol antiemetic effect is its key advantage for outpatient anesthesia - uniquely reduces PONV
13. Fospropofol NOT superior to propofol - slower onset, slower recovery, limited indications; its role is uncertain
14. Six fospropofol PK studies retracted (pre-2010) due to assay inaccuracy - interpret older literature cautiously
15. Propofol does NOT produce analgesia - analgesic adjuncts always required

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