Pharmacodynamics
Gabapentin is a ligand of the α2δ calcium channel subunit.[69][70] α2δ is an auxiliary protein connected to the main α1 subunit (the channel-forming protein) of high voltage activated voltage-dependent calcium channels (L-type, N-type, P/Q type, and R-type).[9] Gabapentin is not a direct channel blocker: it exerts its actions by disrupting the regulatory function of α2δ and its interactions with other proteins. Gabapentin prevents delivery of the calcium channels to the cell membrane, reduces the activation of the channels by the α2δ subunit, decreases signaling leading to neurotransmitters release, and disrupts interactions of α2δ with NMDA receptors, neurexins, and thrombospondins.
[9][10][11] Out of the four known isoforms of α2δ protein, gabapentin binds with similar high
affinity to two:
α2δ-1 and
α2δ-2.
[70] Most of the pharmacological properties of gabapentin are explained by its binding to just one isoform - α2δ-1.
[70][10]
The
endogenous α-amino acids L-leucine and
L-isoleucine, which resemble gabapentin in
chemical structure, bind α2δ with similar affinity to gabapentin and are present in human
cerebrospinal fluid at micromolar concentrations.
[71] They may be the endogenous ligands of the α2δ subunit, and they
competitively antagonize the effects of gabapentin.
[71][72] Accordingly, while gabapentin has
nanomolar affinity for the α2δ subunit, its potency
in vivo is in the low
micromolar range, and competition for binding by endogenous L-amino acids is likely to be responsible for this discrepancy.
[10]
Gabapentin is a
potent activator of voltage-gated potassium channels KCNQ3 and KCNQ5, even at low nanomolar concentrations. However, this activation is unlikely to be the dominant mechanism of gabapentin's therapeutic effects.
[73]
Despite the fact that gabapentin is a structural
GABA analogue, and in spite of its name, it does not bind to the
GABA receptors, does not convert into
GABA or another
GABA receptor agonist in vivo, and does not modulate GABA
transport or
metabolism.
[69]
Pharmacokinetics
Gabapentin is
absorbed from the
intestines by an
active transport process mediated via an
amino acid transporter, presumably,
LAT2.
[74] As a result, the
pharmacokinetics of gabapentin is dose-dependent, with diminished
bioavailability and delayed peak levels at higher doses.
[70]
The
oral bioavailability of gabapentin is approximately 80% at 100 mg administered three times daily once every 8 hours, but decreases to 60% at 300 mg, 47% at 400 mg, 34% at 800 mg, 33% at 1,200 mg, and 27% at 1,600 mg, all with the same dosing schedule.
[5][75] Drugs that increase the transit time of gabapentin in the
small intestine can increase its oral bioavailability; when gabapentin was co-administered with oral
morphine, the oral bioavailability of a 600 mg dose of gabapentin increased by 50%.
[75]
Gabapentin at a low dose of 100 mg has a
Tmax (time to
peak levels) of approximately 1.7 hours, while the Tmax increases to 3 to 4 hours at higher doses.
[70] Food does not significantly affect the Tmax of gabapentin and increases the Cmax and
area-under-curve levels of gabapentin by approximately 10%.
[75]
Gabapentin can cross the
blood–brain barrier and enter the
central nervous system.
[69] Gabapentin concentration in
cerebrospinal fluid is approximately 9-14% of its
blood plasma concentration.
[75] Due to its low
lipophilicity,
[75] gabapentin requires active transport across the blood–brain barrier.
[76][69][77][78] The
LAT1 is highly expressed at the blood–brain barrier
[79] and transports gabapentin across into the
brain.
[76][69][77][78] As with intestinal absorption mediated by an
amino acid transporter, the transport of gabapentin across the blood–brain barrier by LAT1 is saturable.
[76] Gabapentin does not bind to other drug transporters such as
P-glycoprotein (ABCB1) or
OCTN2 (SLC22A5).
[76] It is not significantly
bound to plasma proteins (<1%).
[75]
Gabapentin undergoes little or no
metabolism.
[70][75]
Gabapentin is
eliminated renally in the
urine.
[75] It has a relatively short
elimination half-life, with the reported average value of 5 to 7 hours.
[75] This value changes with increasing doses, from 5.4 hours for a 200 mg single dose, to 8.3 hours for a 1,400 mg dose.
[80] Because of its short elimination half-life, gabapentin must be administered 3 to 4 times per day to maintain therapeutic levels.
[81] Gabapentin XR (brand name Gralise) is taken once a day.
[82]