13.4: Pharmacokinetics in Geriatric Patients: Effect of Age on Drug Absorption

As individuals age, their body's physiology evolves, affecting drug pharmacokinetics. The most apparent changes occur in the gastrointestinal tract, where an increase in gastric pH, a delay in gastric emptying, and a reduction in gastrointestinal motility are observed. Remarkably, these changes do not substantially modify the absorption of orally administered drugs, particularly those absorbed via passive diffusion.

Transdermal drug delivery emerges as a highly viable method for older adults due to its simplicity and the potential for minimizing adverse effects, making it especially suitable for managing chronic pain and neurological conditions. Additional advantages include prolonged therapeutic effects and controlled drug release from the patch. Age-associated skin hydration and lipid content modifications enhance the stratum corneum's barrier function, particularly affecting hydrophilic compounds, whereas lipophilic drugs maintain their absorption efficiency. Studies indicate that age does not significantly alter drug absorption through transdermal systems, though individual drugs like fentanyl may require dosage adjustments for the elderly.

The process of subcutaneous drug absorption, relying on vascular capillaries and lymphatic channels, is also subject to the impact of aging. The molecular size of a drug primarily determines its path—either via capillaries or the lymphatic system—with the skin's blood supply and lymphatic drainage undergoing changes that can influence this route.

Pulmonary pharmacokinetics also evolve with age, as a decrease in alveolar surface area, lung elasticity, capillary volume, and an increase in pulmonary residual volume all contribute to altered drug absorption. Inhaled drugs, including anesthetics like isoflurane and sevoflurane, may require adjusted doses in the elderly to achieve the desired therapeutic effect.

Research on intramuscular drug absorption suggests that it is quite similar to the subcutaneous route. Evidence indicates that the absorption of drugs like diazepam and midazolam remains consistent across different ages.

Ocular drug delivery is not exempt from the effects of aging, with changes in the cornea and Bruch's membrane potentially influencing drug permeability. This is particularly significant for older patients battling ocular diseases such as macular degeneration and glaucoma.

Understanding these age-related changes is essential for optimizing drug therapy in the elderly, ensuring efficacy, and reducing adverse effects.

As people age, various changes in body systems, including the gastrointestinal tract, skin, eyes, and pulmonary system, can affect drug absorption and efficacy.

The gastrointestinal tract experiences increased gastric pH, delayed emptying, and decreased motility but does not typically alter oral drug absorption.

Transdermal drug delivery is optimal for older individuals, offering unaffected drug absorption and reduced side effects.

In contrast, subcutaneous absorption is influenced by changes in vascular capillaries and lymphatic channels, with drug molecule size being a key determinant of passage.

Older adults have altered lung anatomy and physiology, including enlarged alveoli, varied elasticity, and increased residual volume, all of which can influence the absorption of inhaled drugs.

For instance, they require lower doses of inhaled anesthetic drugs than young adults.

Lastly, for ocular drugs, age-related alteration in the choroid and Bruch’s membrane thickness may affect drug permeability from the sub-conjunctival space into the vitreous humor.