Section 2.9: Non-linear and Multi-compartment Pharmacokinetics

Non-linear and Multi-compartment Pharmacokinetics

While one-compartment linear models are useful, many drugs clinically exhibit complex behaviors. These fall into two main categories: non-linear (saturable) kinetics, where pharmacokinetic parameters change with the administered dose, and multi-compartment kinetics, where drugs distribute into different tissues at different rates rather than equilibrating instantaneously.

Non-linear Pharmacokinetics (Michaelis-Menten Kinetics)

In linear pharmacokinetics, clearance ($Cl$) and half-life ($t_{1/2}$) are constant, meaning that doubling the dose results in an exact doubling of the steady-state plasma concentration. However, for drugs that undergo capacity-limited elimination, the metabolic enzymes or transport proteins can become saturated at therapeutic concentrations. This behavior is described by the Michaelis-Menten equation:

$-\frac{dC}{dt} = \frac{V_{max} \cdot C}{K_m + C}$

where $V_{max}$ is the maximum rate of drug elimination (metabolism) in $\text{mg/day}$, $K_m$ is the Michaelis constant (in $\text{mg/L}$), representing the concentration at which the rate of elimination is exactly half of $V_{max}$, and $C$ is the plasma concentration of the drug.

Clinical Implications of Saturation Kinetics

• First-Order Kinetics ($C \ll K_m$): At low concentrations, the equation simplifies to $-dC/dt \approx (V_{max} \cdot C) / K_m$. The rate of elimination is proportional to concentration; clearance is constant, and kinetics are linear.
• Mixed-Order Kinetics ($C \approx K_m$): The rate of elimination is no longer directly proportional to concentration. The clearance decreases as the concentration rises.
• Zero-Order Kinetics ($C \gg K_m$): At high concentrations, the enzymes are fully saturated, and the equation simplifies to $-dC/dt \approx V_{max}$. The drug is eliminated at a constant rate regardless of concentration. Clearance decreases and half-life increases with dose.

Phenytoin: The Classic Example

Phenytoin is the primary clinical example of Michaelis-Menten kinetics. The therapeutic range of phenytoin is $10\text{ to }20\text{ mcg/mL}$, which overlaps with its typical $K_m$ value ($4\text{ to }6\text{ mcg/mL}$). Thus, phenytoin metabolism is saturated within its normal clinical range. A small dose adjustment can lead to a massive, non-linear spike in serum concentration, leading to toxicity (nystagmus, ataxia, slurred speech).

Winter-Tozer Albumin Correction: In patients with hypoalbuminemia (serum albumin $< 3.5\text{ g/dL}$) or end-stage renal disease, phenytoin protein binding is reduced, raising the active free fraction. The measured total phenytoin level must be adjusted using the Winter-Tozer equation:

$C_{adjusted} = \frac{C_{measured}}{0.2 \cdot \text{Albumin} + 0.1}$

Dosing Calculations for Phenytoin: To determine the steady-state concentration ($C_{ss}$) achieved by a daily dose rate ($R$, in $\text{mg/day}$ of active drug):

$R = \frac{V_{max} \cdot C_{ss}}{K_m + C_{ss}}$

To solve for the steady-state concentration ($C_{ss}$):

$C_{ss} = \frac{K_m \cdot R}{V_{max} - R}$

Note on Salt Factor ($S$): Phenytoin sodium capsules have a salt factor of $S = 0.92$, meaning only $92%$ of the dose is active phenytoin. Free phenytoin acid has $S = 1.0$. The active daily dose rate is $R = \text{Dose} \cdot S \cdot F$.

Phenytoin Loading Dose Calculation:

$\text{Loading Dose} = \frac{V_d \cdot (C_{target} - C_{initial})}{S \cdot F}$

Multi-compartment Pharmacokinetics

In a multi-compartment model (commonly a two-compartment model), the body is divided into:

1. Central Compartment: Consists of highly perfused tissues (blood, heart, liver, kidneys, lungs) that equilibrate with the drug rapidly.
2. Peripheral Compartment: Consists of poorly perfused tissues (skeletal muscle, adipose tissue, skin, bone) that equilibrate with the drug slowly.

Biphasic Concentration Decline

Following an intravenous bolus of a multi-compartment drug, a semi-log plot of plasma concentration versus time exhibits a biphasic decline:

• Distribution ($\alpha$) phase: A rapid initial drop in plasma concentration as the drug transfers from the central compartment into the peripheral compartment.
• Elimination ($\beta$) phase: A slower terminal decline where equilibrium between compartments is reached, and the decline in plasma concentration is driven primarily by drug elimination from the central compartment.

Clinical Monitoring Considerations (TDM)

For multi-compartment drugs like digoxin, vancomycin, and lithium, the distribution phase is prolonged.

• If a blood sample is drawn too early (during the $\alpha$-phase), the measured plasma concentration will be extremely high and will not correlate with the drug concentration at the site of action.
• Digoxin: Distribution takes $6\text{ to }8\text{ hours}$. Serum levels must be drawn at least $6\text{ to }8\text{ hours}$ (ideally $12\text{ hours}$) post-dose.
• Vancomycin: Distribution takes $1\text{ to }2\text{ hours}$. Trough concentrations are drawn within $30\text{ minutes}$ prior to the next dose.

Step-by-Step Worked Clinical Example

A $70\text{ kg}$ female patient is currently taking Phenytoin Sodium capsules ($S = 0.92$) at a dose of $300\text{ mg/day}$ ($F = 1.0$). Her steady-state phenytoin concentration is measured to be $10\text{ mcg/mL}$ ($10\text{ mg/L}$). Her estimated Michaelis constant ($K_m$) is $5\text{ mg/L}$.

1. Calculate the active daily drug rate ($R$) currently administered: $R = 300\text{ mg/day} \cdot 0.92 \cdot 1.0 = 276\text{ mg/day}$

2. Calculate the patient's individual maximum metabolic rate ($V_{max}$): $V_{max} = \frac{R \cdot (K_m + C_{ss})}{C_{ss}} = \frac{276\text{ mg/day} \cdot (5\text{ mg/L} + 10\text{ mg/L})}{10\text{ mg/L}} = 414\text{ mg/day}$

3. Determine the new Phenytoin Sodium dose required to achieve a target $C_{ss}$ of $15\text{ mcg/mL}$: $R_{new} = \frac{V_{max} \cdot C_{target}}{K_m + C_{target}} = \frac{414\text{ mg/day} \cdot 15\text{ mg/L}}{5\text{ mg/L} + 15\text{ mg/L}} = 310.5\text{ mg/day}$ Convert the active rate to the Phenytoin Sodium capsule dose: $\text{New Capsule Dose} = \frac{R_{new}}{S} = \frac{310.5\text{ mg/day}}{0.92} \approx 337.5\text{ mg/day}$ Clinical Recommendation: The pharmacist would round this to $340\text{ mg/day}$ (administered as a combination of $300\text{ mg}$ and $40\text{ mg}$ capsules).