Therapeutic Drug Monitoring (TDM) - Definition and Overview

WHO / Authoritative Definition

"The measurement of drugs in biological fluids (most often blood, plasma, or serum) at timed intervals, to maintain drug concentrations within a target therapeutic range - the concentration at which maximal benefit and minimal toxicity are expected for most patients."

1. Measurement - quantitative assay of drug/metabolite concentrations
2. Timing - specimens collected at specific intervals relative to dosing
3. Purpose - maintaining levels within a defined therapeutic window (above minimum effective concentration, below toxic threshold)

Expanded Clinical Definition

"The measurement of serum levels of many therapeutic drugs administered to patients - both because of the possible toxic side effects of many medications, and because lack of patient compliance results in subtherapeutic levels of the drug, requiring intervention."

• Henry's Clinical Diagnosis and Management by Laboratory Methods

• Goodman & Gilman's Pharmacological Basis of Therapeutics describes TDM mathematically as using: CL/F(patient) = Dosing rate / Css (measured), then adjusting maintenance doses accordingly.

Key Principles of TDM

When TDM is Indicated

• Have a narrow therapeutic index (small difference between effective and toxic doses)
• Show marked pharmacokinetic variability between patients
• Are used for a prolonged period with changing patient physiology
• Have effects that are difficult to distinguish clinically from disease activity

• Antiepileptics: phenytoin, carbamazepine, valproate, lamotrigine
• Antibiotics: aminoglycosides (amikacin, gentamicin), vancomycin
• Immunosuppressants: tacrolimus, sirolimus, cyclosporine
• Cardiovascular: digoxin
• Psychiatric: lithium, clozapine
• Antifungals: voriconazole, posaconazole

Purposes of TDM (Summary)

1. Optimize efficacy - ensure concentrations are above the minimum effective threshold
2. Minimize toxicity - prevent accumulation above toxic concentrations
3. Monitor compliance/adherence - detect subtherapeutic levels due to non-adherence
4. Guide dose adjustment - during changes in renal/hepatic function, age, or drug interactions
5. Confirm steady state - verify that dosing has achieved stable therapeutic levels
6. Individualize therapy - account for genetic and physiologic variability (pharmacogenomics)

Timing of Sampling

• Trough (pre-dose) samples: taken just before the next planned dose; best for assessing drug accumulation and adjusting maintenance doses
• Peak (post-dose) samples: taken shortly after a dose to assess whether efficacious concentrations are being reached
• Both peak and trough values together provide the most complete pharmacokinetic picture

Therapeutic Index (TI) - Definition

WHO / Standard Pharmacological Definition

"The therapeutic index is a relative measure of the toxicity or safety of a drug, defined as the ratio of the median toxic dose (TD50) to the median effective dose (ED50)."

The Formula

What TI Means in Practice

"The therapeutic window is the range of plasma drug concentrations between the minimum concentration for clinical efficacy and minimum concentration for toxicity. The therapeutic index is the ratio of these concentrations."

• A high TI = wide margin between effective and toxic doses = safer drug (e.g., penicillin)
• A low / narrow TI = small margin between effective and toxic doses = requires close monitoring (e.g., digoxin, lithium, warfarin, phenytoin)

Visual Illustration

Clinical Significance - High vs. Low TI

"When the therapeutic index is high, as it is for haloperidol, it is reflected by the wide range of dosages in which that drug is prescribed. Conversely, the therapeutic index for lithium is relatively low, thus requiring careful monitoring of serum lithium levels."

Limitations of the TI

1. LD50 is derived from animal studies and may not accurately reflect human toxicity
2. Does not account for idiosyncratic reactions or genetic variation (pharmacogenomics)
3. Efficacy and toxicity both show individual variation not captured by population-level ratios
4. Does not reflect different types of toxicity (e.g., organ-specific vs. systemic)

Drugs with Narrow Therapeutic Index (Require TDM)

• Warfarin, digoxin, lithium, phenytoin, carbamazepine, valproate
• Aminoglycosides (gentamicin, amikacin), vancomycin
• Immunosuppressants (tacrolimus, cyclosporine, sirolimus)
• Antifungals (voriconazole), antineoplastics (5-fluorouracil, methotrexate)
• Theophylline, clozapine

Clinical Pharmacology - WHO Definition

The Official WHO Definition (1970)

"Clinical Pharmacology is the scientific study of drugs in humans."

Expanded WHO / Authoritative Definitions

"Clinical pharmacology takes scientific data on drugs into rational patient care - encompassing drug discovery and development, pharmacokinetics, pharmacodynamics, drug interactions, adverse drug reactions, and the promotion of safer and more effective use of drugs."

"Clinical pharmacology is a discipline that teaches, does research, frames policy, gives information and advice about the actions and proper uses of medicines in humans, and implements that knowledge in clinical practice."

"Two important goals of clinical pharmacology are: (1) to provide a description of conditions under which drug actions vary among human subjects; and (2) to determine mechanisms underlying this variability, with the goal of improving therapy with available drugs as well as pointing to mechanisms whose targeting by new drugs may be effective in the treatment of human disease."

Core Scope of Clinical Pharmacology

Modern WHO Vision for Clinical Pharmacology

1. Teach - train safe prescribers in medical schools and postgraduate programs
2. Research - conduct drug trials and mechanistic studies in humans
3. Frame policy - advise governments on national drug policies
4. Provide information - give unbiased, evidence-based drug advice to regulators, ethics committees, and pricing bodies
5. Support clinical care - manage patients with complex polypharmacy, comorbidities, and altered pharmacokinetics
6. Develop medicines policy - advocate drug quality, equitable access, and rational use

The Bridge Role of Clinical Pharmacology

"Modern clinical pharmacology aims to replace empiricism in the use of drugs with therapy based on in-depth understanding of factors that determine an individual's response to drug treatment. Molecular pharmacology, pharmacokinetics, genetics, clinical trials, and the educated prescriber all contribute to this process."

Basic Pharmacology ←→ CLINICAL PHARMACOLOGY ←→ Clinical Medicine
      (Lab/Animal)           (Humans)              (Patient Care)

Key Distinguishing Features from Basic Pharmacology

Alpha Blockers (Alpha-Adrenergic Antagonists)

Definition

Classification

Mechanism of Action

"α-Adrenergic blocking agents antagonize the subtype(s) of α-adrenergic receptors. Drugs that block α1-adrenoceptors significantly affect blood pressure. Because normal sympathetic control of the vasculature occurs in large part through agonist actions on α1-adrenergic receptors, blockade reduces the sympathetic tone of blood vessels, resulting in decreased peripheral vascular resistance and a subsequent reduction in blood pressure. This decreased blood pressure induces reflex tachycardia."

"Alpha1-receptor selectivity allows norepinephrine to exert unopposed negative feedback (mediated by presynaptic α2 receptors) on its own release. In contrast, phentolamine blocks both presynaptic and postsynaptic α receptors - reflex activation of sympathetic neurons produces greater release of transmitter onto β receptors and correspondingly greater cardioacceleration."

Epinephrine Reversal

• Epinephrine's α-mediated vasoconstriction is blocked
• Epinephrine's β2-mediated vasodilation is unmasked
• Net result: Blood pressure FALLS instead of rises - this is called "epinephrine reversal"
• Norepinephrine (lacks significant β2 effect on vasculature): actions are diminished but not reversed
• Isoproterenol (pure β agonist): completely unaffected by alpha blockade

Individual Drug Profiles

1. Phenoxybenzamine

• Type: Irreversible, non-selective (α1 + α2) blocker (haloalkylamine)
• Duration: >24 hours (irreversible)
• Uses: Pheochromocytoma (pre-operative preparation, inoperable disease), hypertensive crisis from sympathomimetics, autonomic hyperreflexia in spinal cord injury, neurogenic bladder
• Dose for pheo: 10 mg BD initially, titrated to 40-120 mg/day
• ADRs: Orthostatic hypotension, reflex tachycardia, nasal stuffiness, inhibited ejaculation, GI irritation, arrhythmias

2. Phentolamine

• Type: Reversible, non-selective (α1 + α2), competitive (imidazoline)
• Duration: ~4 hours (IV half-life ~45 min)
• Uses:
  • Short-term management of pheochromocytoma
  • Hypertensive crisis (clonidine withdrawal, tyramine-MAOI interaction)
  • Prevention of dermal necrosis after norepinephrine extravasation
  • Reversing soft-tissue local anaesthetic effect in dental procedures
  • Pseudo-obstruction of bowel in pheo
• ADRs: Severe hypotension with rapid IV infusion, reflex tachycardia

3. Prazosin, Terazosin, Doxazosin (Selective α1 Blockers)

• Type: Selective α1 (competitive), not α2
• Mechanism: Relax arterial AND venous smooth muscle → reduce peripheral resistance
• Minimal reflex tachycardia (α2 presynaptic feedback intact)
• Uses: Hypertension (second-line), BPH (especially in men with both conditions)
• Pharmacokinetics:

4. Tamsulosin, Alfuzosin, Silodosin (Uroselective α1A Blockers)

• Selective for α1A receptors in prostate/bladder > α1B in vessels
• Less blood pressure lowering effect → safer in normotensive BPH patients
• Primary use: BPH (improve urine flow, reduce outlet resistance)
• Also used as medical expulsive therapy for ureteric calculi

5. Yohimbine (Selective α2 Blocker)

• Blocks presynaptic α2 → increases central sympathetic outflow + NE release
• Raises BP and HR
• Uses: Male erectile dysfunction (limited), orthostatic hypotension
• ADRs: Anxiety, tremor, excess pressor effect if NE transporter blocked

Clinical Uses Summary

Adverse Effects

• Starting at 1/3 to 1/4 the normal dose
• Administering the first dose at bedtime
• Ensuring patient is not salt/volume depleted

Contraindications / Cautions

• Phenoxybenzamine/phentolamine: Caution in cerebrovascular and cardiovascular disease (reflex tachycardia may provoke angina/arrhythmia)
• Note: Beta-blockers must never be started before alpha blockade in pheochromocytoma - doing so causes paradoxical severe hypertension (β vasodilation blocked, leaving α vasoconstriction unopposed)
• Tamsulosin: Warn patients about IFIS before any cataract surgery
• All α1 blockers + PDE-5 inhibitors (sildenafil, tadalafil): Dangerous additive hypotension