Huntington Disease Triplet Repeat: Pharmacokinetic Profiling (Geriatric Update)

■ LECTURE OVERVIEW: Huntington Disease is a progressive, autosomal dominant neurodegenerative disorder characterized by core motor, cognitive, and psychiatric symptoms. ■ GENETIC DETAILS: 1. Triplet Expansion: Caused by an expanded CAG trinucleotide repeat sequence in the HTT gene on the short arm of chromosome 4 (4p16.3). This gene encodes the huntingtin protein. 2. CAG Codon coding: The CAG codon codes for the amino acid Glutamine. Thus, the mutation creates an abnormally long polyglutamine (polyQ) tract on the huntingtin protein. 3. Toxic Gain of Function: The polyglutamine expansion induces conformational changes, causing mutated huntingtin to misfold, resist proteasomal degradation, and form toxic intracellular aggregates. 4. Striatal Nuclear Death: Aggregates damage transcription factors (like CREB-binding protein) and disrupt mitochondrial function, selectively destroying the GABAergic medium spiny neurons in the caudate nucleus and putamen (striatum). ■ PHARMACOKINETIC & PHARMACODYNAMIC ATTRIBUTES: Absorption and steady-state kinetics display high variability based on plasma protein binding levels, tissue volume of distribution (Vd), and hepatic CYP450 microsomal enzymatic clearance indices. ■ GERIATRIC PHYSIOLOGIC ADJUSTMENTS: Older patients display reduced physiological reserves, altered muscle-to-fat distributions, and distinct renal filtration profiles. [HY-BOARD-1132]