The Journey of Gabapentin

Gabapentin is absorbed from the small intestine by active transport
via the large neutral amino acid transporter LAT1 (SLC7A5) and LAT2 (SLC7A8) — the same
transporters used by leucine, isoleucine, and other branched-chain amino acids. This transport
system is saturable, producing a non-linear (dose-limited) pharmacokinetic profile: bioavailability
is approximately 60% at 300 mg three times daily but decreases to approximately 35% at 1,600 mg
three times daily. High-protein meals (competing amino acids) can reduce gabapentin absorption.
Peak plasma concentrations occur within 2-3 hours. There is no significant hepatic first-pass
metabolism. The drug's zwitterionic nature (pKa 3.68 for carboxylate, 10.7 for amine) makes it
essentially non-lipophilic (logP ≈ -1.1) and entirely dependent on transporter-mediated absorption.
Gabapentin enacarbil (a prodrug) uses a different transporter (MCT-1) for improved and linear
bioavailability.

Gabapentin distributes to the extracellular fluid and CNS with
a volume of distribution of approximately 58 L (0.8 L/kg). The drug crosses the blood-brain barrier
via the same LAT1 transporter expressed on brain capillary endothelium. Plasma protein binding is
less than 3% — essentially unbound in plasma. This minimal protein binding means that drug-drug
interactions via protein displacement do not occur, and concentrations in CSF are approximately
20% of plasma concentrations. Gabapentin is also present in breast milk (approximately 1-3.8 mg/L),
though infant exposure is considered low. Tissue concentrations are highest in the CNS relative to
other peripheral tissues. The non-lipophilic nature prevents significant accumulation in fat or
other lipid-rich compartments.

Despite its structural resemblance to GABA (gamma-aminobutyric acid),
gabapentin does not bind GABA-A or GABA-B receptors, does not inhibit GABA transaminase, and does
not affect GABA transport or synthesis. The primary molecular target is the alpha-2-delta (α2δ)
auxiliary subunit of voltage-gated calcium channels (VGCCs), specifically the α2δ-1 and α2δ-2
isoforms. The α2δ subunit regulates calcium channel trafficking to the plasma membrane and modulates
channel gating. Gabapentin binding to α2δ-1 (at a site homologous to the binding site for arginine
and leucine) reduces the surface expression of VGCCs — particularly N-type (Cav2.2) and P/Q-type
(Cav2.1) channels at presynaptic terminals. Reduced calcium influx during action potentials decreases
release of excitatory neurotransmitters (glutamate, substance P, norepinephrine) from hyperexcited
nociceptive and epileptic neurons, without affecting normal neurotransmission at physiological
firing rates — explaining the use-dependent selectivity for pathologically active circuits.

Gabapentin is not metabolized in humans. The drug is excreted
entirely unchanged. It is not a substrate or inhibitor of any cytochrome P450 enzyme, and does not
affect hepatic microsomal enzyme activity. This metabolic inertness makes gabapentin essentially
free of pharmacokinetic drug-drug interactions through CYP pathways, representing a significant
clinical advantage in polypharmacy patients. There are no active or inactive metabolites to monitor.
The absence of hepatic metabolism means that dose adjustment for hepatic impairment is not required.
However, since elimination is entirely renal, gabapentin accumulates significantly in renal
impairment, requiring proportional dose reduction based on creatinine clearance.

Gabapentin is eliminated entirely by renal excretion as unchanged
drug, with clearance proportional to creatinine clearance. In patients with normal renal function,
the elimination half-life is 5-7 hours, requiring dosing 3 times daily. The half-life extends
dramatically in renal impairment: to approximately 52 hours in severe renal failure (CrCl <30 mL/min)
and up to 132 hours in patients on hemodialysis (requiring supplemental doses after each session).
Gabapentin is efficiently removed by hemodialysis. Dose adjustments for renal impairment are well-
characterized in the prescribing information and critical for safety. The saturable absorption
and renal-only elimination create a narrow effective dose range, and doses should be reduced
proportionally with declining renal function.

Gabapentin is FDA-approved for postherpetic neuralgia (PHN) and
adjunctive therapy for partial-onset seizures. Off-label uses include neuropathic pain, fibromyalgia,
restless legs syndrome, alcohol withdrawal, and anxiety disorders — many of which have strong evidence
but are regulatory off-label. Adverse effects include somnolence, dizziness, ataxia, and peripheral
edema. Misuse and diversion have emerged as serious public health concerns; gabapentin can potentiate
opioid respiratory depression and is associated with overdose deaths in combination with opioids,
leading several US states to classify it as a Schedule V controlled substance. Drug-drug interactions
are primarily pharmacodynamic (CNS depression, respiratory depression with opioids and benzodiazepines)
rather than pharmacokinetic.