Therapeutic Drug Monitoring: Principles, Applications, and Inter-Individual Variability

Overview of Therapeutic Drug Monitoring (TDM) and the Case Failure

• This lecture (UniSA_PHAR2006) focuses on Therapeutic Drug Monitoring (TDM), specifically picking up from a practical session where a patient was treated with a drug using a designed dosage regimen.

• Despite following all correct procedures—calculating renal and hepatic clearance based on the patient's individual factors such as Glomerular Filtration Rate ($GFR$), creatinine clearance, and body weight—the regimen failed.

• The specific failure observed was that the trough concentrations of the drug were too high. This caused the patient's drug levels to fluctuate around an average that was not centered in the therapeutic window, leading to toxicity.

The Vicious Cycle of Nephrotoxicity and Drug Clearance

• The speaker describes a feedback loop or "vicious cycle" that occurs when concentrations result in toxicity:

  • High trough concentrations cause increased toxicity.

  • In this case, the toxicity resulted in damage to the kidneys.

  • Kidney damage decreased the renal clearance of the drug (as the drug was primarily renally cleared).

  • A decrease in clearance leads to higher concentrations of the drug when the next dose is administered.

  • The higher concentrations increase the level of toxicity, causing further reduction in clearance.

  • This cycle continues, with each round increasing concentrations and subsequent damage.

Limitations of Population-Based Individualization

• The initial regimen failed even though it was individualized based on specific data about the man (weight, $GFR$) because it was still fundamentally based on the "average person."

• Inter-individual variability (or "into individual variability") means that two people with the same weight or measured kidney function can process drugs differently. For example:

  • A person might only have $60 \%$ of their measured $GFR$ as actual renal clearance for a specific drug.

  • A person might have less than the normal amount of non-renal clearance for their given body weight.

• Even recalculating the creatinine clearance using new serum creatinine levels after damage has occurred is insufficient for designing a perfect regimen because the patient's unique kinetics still deviate from the population average.

Determining Individual Clearance in TDM

• To design a truly unique regimen, practitioners must determine the patient's actual, individual clearance rather than estimating it.

• Process for determining individual clearance:

  • Take a couple of blood samples around the middle of a dosing interval.

  • Measure the steady-state average concentration ($C_{ss,avg}$).

  • Use the biological relationship where the steady-state average concentration is determined by the dose rate and the drug's clearance:     $C_{ss,avg} = \frac{\text{Dose Rate}}{\text{Clearance}}$

• Because the dose rate is known and the $C_{ss,avg}$ is measured through TDM, the individual clearance can be calculated precisely.

Definitions and Clinical Requirements for TDM

• Definition: Therapeutic Drug Monitoring (TDM) involves measuring something—usually a drug concentration—and using that measurement to understand the patient's unique kinetics to refine a dosage regimen to meet the specific individual's situation.

• Necessary Conditions for TDM:

  • Narrow Therapeutic Window: If the window were broad, individual kinetic variability would not matter as much because most people would remain within the safe/effective range. TDM is necessary when the window is narrow.

  • Substantial Pharmacokinetic Variability: Even with a narrow window, TDM is only required if there is significant variability in how individuals handle the drug. If everyone were "average," they would all stay within the window without monitoring.

  • Non-measurable Effects: TDM is used when the effect of the drug cannot be easily or directly measured. In these cases, the drug concentration serves as a surrogate for the likelihood of a positive therapeutic outcome.

Prophylactic and Preventative Applications

• Prophylactic Therapy: TDM is essential for drugs used to prevent illness (prophylaxis). The absence of illness does not necessarily mean the drug is at an effective dose; it might just mean the illness hasn't occurred yet. TDM confirms the concentration is in the safe and effective range.

• Toxicity Prevention: TDM is used to proactively check concentrations to ensure dosing is correct, rather than waiting for physical signs of toxicity to manifest before taking action.

The Rationale and Interdisciplinary Team Effort

• Concentration-Effect Relationship: TDM relies on the principle that the concentration of the drug in the plasma reflects the concentration at the target site and is the driver of the clinical effect.

• The TDM Process Loop:

  1. The patient is prescribed an initial dose regimen.

  2. Nursing staff take blood samples from the patient.

  3. Laboratory scientists measure the specific drug concentration in the samples.

  4. A multidisciplinary team interprets the information. This team includes the prescriber, the clinical pharmacologist (noted as a "very useful and valuable person" in hospitals), and the pharmacist.

  5. The dose regimen is refined based on this collective interpretation.

  6. Ongoing monitoring ensures the patient remains within the target therapeutic range.

• Economic Considerations: TDM is notably costly compared to not monitoring concentrations. However, the costs are often offset by improved patient outcomes, increased wellness, and the avoidance of expensive toxicities.