Polypharmacy Management, Pharmacokinetics/Pharmacodynamics in Aging & Deprescribing Protocols

Polypharmacy Management, Pharmacokinetics/Pharmacodynamics in Aging & Deprescribing Protocols

Polypharmacy and Hyperpolypharmacy

Managing multimorbidity in older adults frequently leads to complex medication regimens. Polypharmacy is clinically defined as the routine, concurrent use of five or more medications (including prescription drugs, over-the-counter [OTC] medications, dietary supplements, and herbal products). Hyperpolypharmacy is defined as the concurrent use of ten or more medications.

Polypharmacy is an independent risk factor for adverse drug reactions (ADRs), harmful drug-drug interactions, non-adherence, medication errors, cognitive impairment, functional decline, fall-related fractures, acute hospitalizations, and all-cause mortality. As the number of concurrent medications increases, the probability of an adverse drug interaction approaches 100% when a patient takes eight or more drugs.

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The Prescribing Cascade Phenomenon

A major driver of polypharmacy is the prescribing cascade. A prescribing cascade occurs when an adverse drug reaction caused by one medication is misdiagnosed as a new, independent medical condition, prompting the clinician to prescribe a second medication to treat the drug-induced symptom. The second drug often produces additional side effects, triggering further prescriptions.

Classic Clinical Examples of Prescribing Cascades:

1. Dihydropyridine CCB Cascade: An NP prescribes amlodipine for hypertension. The patient develops bilateral lower-extremity peripheral edema. The clinician misinterprets the edema as worsening heart failure and prescribes furosemide. The diuretic subsequently causes hypokalemia, hyperuricemia, or orthostatic hypotension.
2. Cholinesterase Inhibitor Cascade: A patient with Alzheimer's dementia is started on donepezil. Donepezil causes bladder detrusor hyperactivity and urinary urge incontinence. The provider prescribes oxybutynin (an anticholinergic). Oxybutynin crosses the blood-brain barrier and antagonizes donepezil, causing rapid cognitive deterioration and delirium.
3. NSAID Cascade: A patient takes chronic oral ibuprofen for knee osteoarthritis. The NSAID induces fluid retention and elevates blood pressure. The clinician misdiagnoses essential hypertension and adds an antihypertensive agent (e.g., lisinopril).
4. Dopamine Antagonist Cascade: A patient prescribed haloperidol or metoclopramide develops drug-induced parkinsonism. The provider misdiagnoses Parkinson's disease and initiates levodopa/carbidopa.

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Age-Related Pharmacokinetics (PK) Changes

Pharmacokinetics describes what the body does to a drug (Absorption, Distribution, Metabolism, and Excretion [ADME]). Normal physiological aging significantly alters each PK phase.

1. Absorption

• Physiological Changes: Decreased gastric acid secretion (elevated gastric pH), reduced mucosal surface area, delayed gastric emptying, and decreased splanchnic blood flow.
• Clinical Impact: Passive absorption of most oral drugs remains intact, but drugs requiring an acidic environment (e.g., iron salts, calcium carbonate) exhibit decreased bio-availability.

2. Distribution

• Physiological Changes: Relative increase in total body fat (mass increases by 20–40%), decrease in total body water (water decreases by 10–15%), and reduction in serum plasma albumin.
• Fat-Soluble Drugs: Lipophilic medications (e.g., diazepam, amiodarone) have an increased volume of distribution ($V_d$). These drugs accumulate in adipose tissue, significantly prolonging elimination half-life ($t_{1/2}$) and causing protracted sedation.
• Water-Soluble Drugs: Hydrophilic medications (e.g., lithium, digoxin, gentamicin) have a decreased volume of distribution ($V_d$), leading to higher peak serum concentrations and acute toxicity.
• Protein Binding: Highly protein-bound drugs (e.g., warfarin, phenytoin) have less bound drug and a higher proportion of active "free" drug in circulation when serum albumin is low (<3.5 g/dL).

3. Metabolism

• Physiological Changes: Hepatic mass decreases by 20–30%, and hepatic blood flow declines by 20–40%.
• Phase I vs. Phase II Metabolism: Phase I reactions (CYP450 oxidation, reduction) decline significantly, prolonging half-lives of drugs like diazepam and warfarin. Phase II reactions (glucuronidation, conjugation) are preserved in healthy aging. Benzodiazepines cleared via direct Phase II glucuronidation—recalled by the mnemonic LOT (Lorazepam, Oxazepam, Temazepam)—are preferred in geriatrics.

4. Excretion

• Physiological Changes: Progressive reduction in renal mass, renal blood flow, and GFR (eGFR declines ~1 mL/min/year after age 40).
• Serum Creatinine Pitfall: Serum creatinine alone is an unreliable indicator of renal function in older adults due to age-related loss of muscle mass (sarcopenia). Clinicians must calculate Cockcroft-Gault CrCl or CKD-EPI eGFR to adjust renally cleared drugs (gabapentin, DOACs, digoxin).

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Age-Related Pharmacodynamics (PD) Changes

Pharmacodynamics describes what the drug does to the body. Older adults demonstrate altered end-organ sensitivity and reduced homeostatic compensatory mechanisms.

• CNS Sensitivity: Increased blood-brain barrier permeability and altered receptor density increase CNS sensitivity. Opioids, benzodiazepines, and anticholinergics produce exaggerated sedation, ataxia, and delirium at lower doses.
• Cardiovascular Sensitivity: Blunted baroreceptor reflex responsiveness impairs compensation for postural changes. Vasodilators, alpha-blockers, and diuretics produce severe orthostatic hypotension.

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The 5-Step Evidence-Based Deprescribing Protocol

Deprescribing is the systematic, supervised process of dose reduction or discontinuation of medications that may cause harm, lack clinical efficacy, or no longer align with patient goals.

Step-by-Step Execution:

1. Step 1: Comprehensive Reconciliation: Obtain a complete list of all active medications, including OTCs, herbals, topicals, and eye drops ("brown bag" review).
2. Step 2: Identify PIMs and Prescribing Cascades: Cross-reference active drugs against Beers, STOPP, and ACB scales. Identify unindicated drugs or prescribing cascades.
3. Step 3: Assess Feasibility and Prioritize Targets: Determine which drugs pose the highest risk of acute harm. Engage in shared decision-making with the patient and caregivers to align therapy with goals of care.
4. Step 4: Formulate Individualized Tapering Protocols: Gradually taper drugs associated with withdrawal syndromes:
   • Proton Pump Inhibitors (PPIs): Taper over 4 to 8 weeks (reduce dose by 50% every 2 weeks, then switch to every-other-day dosing) to prevent rebound acid hypersecretion.
   • Beta-Blockers: Taper slowly over several weeks to avoid rebound tachycardia, chest pain, and hypertension.
   • Benzodiazepines & Gabapentinoids: Reduce daily dose by 10% to 25% every 1 to 2 weeks over several months to prevent withdrawal seizures, severe anxiety, and delirium.
5. Step 5: Close Monitoring and Patient Communication: Follow up frequently to evaluate for withdrawal symptoms, disease recurrence, or clinical improvement. Frame deprescribing positively as a proactive measure to optimize vitality and prevent drug-induced illness.