Pharmacological properties : תכונות פרמקולוגיות

Pharmacodynamic Properties

5.1    Pharmacodynamic properties
Pharmacotherapeutic groups: Other antiepileptics ATC code: N03AX12

Mechanism of action
Gabapentin readily enters the brain and prevents seizures in a number of animal models of epilepsy.
Gabapentin does not possess affinity for either GABAA or GABAB receptor nor does it alter the metabolism of GABA. It does not bind to other neurotransmitter receptors of the brain and does not interact with sodium channels. Gabapentin binds with high affinity to the 25 (alpha-2-delta) subunit of voltage-gated calcium channels and it is proposed that binding to the 25 subunit may be involved in gabapentin’s anti-seizure effects in animals. Broad panel screening does not suggest any other drug targets other than 25.
Evidence from several pre-clinical models inform that the pharmacological activity of gabapentin may 
be mediated via binding to 25 through a reduction in release of excitatory neurotransmitters in regions of the central nervous system. Such activity may underlie gabapentin’s anti-seizure activity.
The relevance of these actions of gabapentin to the anticonvulsant effects in humans remains to be established.
Gabapentin also displays efficacy in several pre-clinical animal pain models. Specific binding of gabapentin to the 25 subunit is proposed to result in several different actions that may be responsible for analgesic activity in animal models. The analgesic activities of gabapentin may occur in the spinal cord as well as at higher brain centers through interactions with descending pain inhibitory pathways. The relevance of these pre-clinical properties to clinical action in humans is unknown.

Pharmacokinetic Properties

5.2    Pharmacokinetic properties

Absorption
Following oral administration, peak plasma gabapentin concentrations are observed within 2 to 3 hours. Gabapentin bioavailability (fraction of dose absorbed) tends to decrease with increasing dose.
Absolute bioavailability of a 300 mg capsule is approximately 60%. Food, including a high-fat diet, has no clinically significant effect on gabapentin pharmacokinetics.

Gabapentin pharmacokinetics are not affected by repeated administration. Although plasma gabapentin concentrations were generally between 2 g/mL and 20 g/mL in clinical studies, such concentrations were not predictive of safety or efficacy. Pharmacokinetic parameters are given in Table 3.

Table 3
Summary of gabapentin mean (%CV) steady-state pharmacokinetic parameters following every eight hours administration
Pharmacokinetic           300 mg                      400 mg                        800 mg parameter                 (N = 7)                     (N = 14)                      (N=14) Mean         %CV            Mean           %CV            Mean         %CV Cmax (µg/mL)              4.02         (24)           5.74           (38)           8.71          (29) tmax (hr)                 2.7          (18)           2.1            (54)           1.6           (76) T1/2 (hr)                 5.2          (12)           10.8           (89)           10.6          (41) AUC (0-8)                 24.8         (24)           34.5           (34)           51.4          (27) g hr/mL)
.

Ae% (%)                   NA           NA             47.2           (25)           34.4          (37) Cmax = Maximum steady state plasma concentration tmax = Time for Cmax
T1/2 = Elimination half-life
AUC(0-8) = Steady state area under plasma concentration-time curve from time 0 to 8 hours postdose Ae% = Percent of dose excreted unchanged into the urine from time 0 to 8 hours postdose NA = Not available

Distribution

Gabapentin is not bound to plasma proteins and has a volume of distribution equal to 57.7 litres. In patients with epilepsy, gabapentin concentrations in cerebrospinal fluid (CSF) are approximately 20% of corresponding steady-state trough plasma concentrations. Gabapentin is present in the breast milk of breast-feeding women.

       Biotransformation

There is no evidence of gabapentin metabolism in humans. Gabapentin does not induce hepatic mixed function oxidase enzymes responsible for drug metabolism.

Elimination

Gabapentin is eliminated unchanged solely by renal excretion. The elimination half-life of gabapentin is independent of dose and averages 5 to 7 hours.

In elderly patients, and in patients with impaired renal function, gabapentin plasma clearance is reduced. Gabapentin elimination-rate constant, plasma clearance, and renal clearance are directly proportional to creatinine clearance.

Gabapentin is removed from plasma by haemodialysis. Dosage adjustment in patients with compromised renal function or undergoing haemodialysis is recommended (see section 4.2).

Linearity/non-linearity

Gabapentin bioavailability (fraction of dose absorbed) decreases with increasing dose which imparts non-linearity to pharmacokinetic parameters which include the bioavailability parameter (F) e.g. Ae%, CL/F, Vd/F. Elimination pharmacokinetics (pharmacokinetic parameters which do not include F such as CLr and T1/2), are best described by linear pharmacokinetics. Steady state plasma gabapentin concentrations are predictable from single-dose data.