About

The Phenytoin Maintenance Dose Calculator utilizes Michaelis-Menten kinetics to estimate the daily phenytoin dose required to achieve a desired steady-state serum concentration (Css). This approach is essential because phenytoin exhibits saturable, non-linear pharmacokinetics, meaning that small changes in the maintenance dose can lead to disproportionately large changes in serum levels, especially near or above the upper therapeutic range.

Outputs

Upon calculation, the tool provides the following outputs for clinical consideration:

• Calculated Dose: A precise dose in mg/day based on the input parameters and the Michaelis-Menten formula.
• Recommended Dose: The calculated dose rounded to a clinically practical value, typically to the nearest 25 mg, to align with available dosage forms.
• Suggested Regimen: An example of how the recommended daily dose could be administered using common capsule strengths (e.g., 100 mg capsules).
• Clinical Pearls & Warnings: Context-sensitive alerts, such as a warning for low serum albumin (e.g., <3.5 g/dL) or for high calculated doses (e.g., >400 mg/day), which may increase toxicity risk.

How to Use

To calculate a maintenance dose, enter the following patient-specific and pharmacokinetic parameters:

1. Weight: The patient's total body weight in kilograms (kg) or pounds (lbs).
2. Target Phenytoin Conc. (Css): The desired steady-state serum concentration. The typical therapeutic range for total phenytoin is 10-20 mg/L.
3. Vmax (Max Rate of Metabolism): The maximum rate of drug metabolism. A population average of 7 mg/kg/day is used as a default but should be adjusted based on clinical context (e.g., liver function, interacting drugs).
4. Km (Michaelis-Menten Constant): The drug concentration at which the rate of metabolism is half of Vmax. A population average of 4 mg/L is a common starting point.
5. Phenytoin Formulation (Salt Factor, S): Select the correct formulation, as different forms contain different amounts of active phenytoin acid. The salt factor (S) is 0.92 for sodium salts (IV, capsules) and 1.0 for phenytoin acid (suspension, chewables) and fosphenytoin.
6. Oral Bioavailability (F): The fraction of the dose reaching systemic circulation. For phenytoin, this is generally assumed to be 1.0 (100%).
7. Optional Inputs (Albumin, Age): Entering serum albumin is critical if it is low, as this will trigger a warning regarding the need to consider corrected phenytoin levels. Age can provide additional clinical context.

Dosing Overview

Phenytoin dosing must be carefully individualized. Unlike linear drugs, there is no direct proportionality between dose and serum concentration. Once the metabolic enzymes become saturated, even a small dose increase can cause a large, unpredictable rise in serum levels, leading to toxicity. Initial dosing is often weight-based, but maintenance dosing requires therapeutic drug monitoring.

The standard therapeutic range for total phenytoin is 10 to 20 mg/L (mcg/mL). The free (unbound), active fraction is typically 1 to 2 mg/L. Levels should be drawn at steady state, approximately 5-10 days after a dose change in adults with normal renal and hepatic function.

Switching

When switching between different phenytoin formulations, it is crucial to account for the salt factor (S) to ensure equivalent dosing of the active moiety.

• Phenytoin Sodium (S=0.92) to Phenytoin Acid (S=1.0): To switch from a sodium salt capsule to the acid suspension, the total daily dose should be slightly reduced. For example, 300 mg of phenytoin sodium is equivalent to approximately 276 mg of phenytoin acid.
• IV Fosphenytoin to Oral Phenytoin: Fosphenytoin is dosed in phenytoin sodium equivalents (PE), where 1 mg PE = 1 mg of phenytoin sodium. Therefore, the conversion to oral phenytoin sodium capsules is typically 1:1.

Always re-check serum levels after a formulation switch to confirm the target concentration is maintained.

Missed Dose

For a missed dose of extended-release phenytoin, the patient should take the missed dose as soon as they remember. If it is almost time for the next scheduled dose, they should skip the missed dose and resume their regular dosing schedule. Patients should be instructed not to take two doses at once to "catch up," as this can significantly increase the risk of toxicity.

Safety Alerts

Key Safety Considerations

• Non-Linear Kinetics: Small dose adjustments can lead to disproportionately large changes in serum concentration and toxicity. Dose increases should be made cautiously.
• Low Albumin: In patients with hypoalbuminemia (<3.5 g/dL) or renal impairment, the free fraction of phenytoin is increased. The total level may appear therapeutic, but the patient can be toxic. A corrected phenytoin level should be calculated.
• Drug Interactions: Phenytoin is a potent inducer of CYP3A4 and other enzymes, affecting the metabolism of many drugs. It is also a substrate, and numerous medications can inhibit or induce its metabolism, altering its levels.
• Adverse Effects: Common side effects include nystagmus, ataxia, and drowsiness. Chronic use can lead to gingival hyperplasia, hirsutism, and bone density loss.

FAQ

References

1. DILANTIN (phenytoin sodium) extended capsules [Prescribing Information]. New York, NY: Pfizer Inc.; 2023. Available from fda.gov
2. Cerebyx (fosphenytoin sodium) injection [Prescribing Information]. New York, NY: Pfizer Inc.; 2022. Available from fda.gov
3. Patsalos PN, Spencer EP, Berry DJ. Therapeutic drug monitoring of antiepileptic drugs in epilepsy: a 2018 update. Ther Drug Monit. 2018;40(5):526-548. doi:10.1097/FTD.0000000000000546
4. Winter ME. Basic Clinical Pharmacokinetics. 5th ed. Lippincott Williams & Wilkins; 2010.
5. Tatum WO, et al. Updated status of antiepileptic drug therapeutic drug monitoring and plasma level testing. Epilepsy Currents. 2021;21(3):209-216. doi:10.1177/15357597211005235