Gabapentin

Formulation

The table below describes the formulation of gabapentin and the control morphine sulfate for the following in vivo experiments.

Experiment(s)	Compound	Dose(s) mg/kg	Correction factor	Dose volume mL/kg	Route	Vehicle	Suspension / Solution
Pharmacokinetics	Gabapentin	60, 100	1.00	5	PO	Saline	Solution
Irwin, Rotarod	Gabapentin	30, 60, 100, 300	1.00	5	PO	Saline	Solution
Plantar incision,	Gabapentin	10, 30, 60, 100	1.00	5	PO	Saline	Solution
L5/L6 SNL	Gabapentin	10, 30, 60, 100	1.00	5	PO	Saline	Solution
Plantar incision,	Morphine	6	1.33	1	SC	Saline	Solution
L5/L6 SNL	Morphine	6	1.33	1	SC	Saline	Solution

L5/L6 SNL = L5/L6 spinal nerve ligation; PO = per os; SC = subcutaneous

Results

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Method: Protein Binding

A table shows the results of rapid equilibrium dialysis (RED) and Centrifree centrifugation assays to assess protein binding and percent recovery of gabapentin. Gabapentin levels in plasma and brain were not determined (ND) due to the gabapentin concentration being higher in the buffer compartment than the protein matrix in the RED analysis. In rat plasma in RED, gabapentin had a recovery of 101.3%. In rat brain homogenate in RED, gabapentin had a recovery of 104.1%. In rat plasma in Centrifree, gabapentin was 2.8% bound to protein and 97.2% unbound and had a recovery of 119.3%. In rat brain homogenate in Centrifree, gabapentin was 10.8% bound to protein and 89.2% unbound and had a recovery of 117.6%. Acebutolol, quinidine, and warfarin were included in the study as controls and returned values consistent with literature values in both RED and Centrifree assays.

Method: Pharmacokinetics

Animals were randomly assigned to treatment groups.

Experimenters collecting samples were masked to treatment.

Gabapentin concentrations were measured in serially collected plasma samples and terminally collected brain samples (shown on two graphs) from males and females treated with 60 or 100 mg/kg gabapentin (PO) in a vehicle of saline. There were 4 rats/group for plasma and 3-4 rats/group for brain.

Method: Pharmacokinetics

A table shows pharmacokinetics parameters calculated for gabapentin using non-compartmental analysis in male and female rats treated with gabapentin (60 or 100 mg/kg, PO, vehicle of saline). The means for the 4 groups (males and females treated with 60 mg/kg gabapentin and males and females treated with 100 mg/kg gabapentin, respectively) are as follows. The highest concentration (Cmax) was 101.57, 116.74, 138.62, and 149.75 µM; the time at the maximum concentration (Tmax) was 1.75, 1, 1, and 1.5 hours; the half-life (T1/2) was 3.46, 2.54, 3.71, and 2.91 hours; area under the curve for time 0 to 8 hours was 30349, 29162, 38178, and 39157 minutes*µM; area under the curve for time 0 to infinity was 39041, 33319, 51051, and 47251 minutes*µM; the brain-to-plasma ratio at 2 hours was 0.95, 0.82, 0.93, and 0.99; the brain-to-plasma ratio at 8 hours was 0.96, 1.05, 0.83, and 0.8

Method: Modified Irwin functional observational battery

Animals were balanced across treatment groups based on body weight.

Experimenters assessing behavior were masked to treatment.

Group size was determined using power analysis.

Two reviewers observed and scored 39 behaviors in a modified Irwin test. For each animal, the scores for behavioral observations from each reviewer are summed then averaged for male and female rats (shown on two graphs). Responses are shown at the following time points: baseline (before treatment) and at 1, 2, 4, and 6 hours after treatment with vehicle (saline, delivered PO) or gabapentin (30, 60, 100, or 300 mg/kg, delivered PO). There were 4 rats per group. Kruskal-Wallis tests were performed at each timepoint for each sex, followed by Dunn’s tests when a significant effect was observed. Dunn’s tests found an increase in the number of observations relative to vehicle in males at 2 hours post-treatment with 100 mg/kg gabapentin (p<0.05) and 300 mg/kg gabapentin (p<0.05), at 4 hours post-treatment with 60 mg/kg gabapentin (p<0.05) and 300 mg/kg gabapentin (p<0.05), and at 6 hours post-treatment with 60 mg/kg gabapentin (p<0.01). Dunn’s tests found an increase in the number of observations relative to vehicle in females at 1 hour post-treatment with 30 mg/kg gabapentin (p<0.05) and 100 mg/kg gabapentin (p<0.05); at 4 hours post-treatment with 30 mg/kg gabapentin (p<0.05), 60 mg/kg gabapentin (p<0.05), and 300 mg/kg gabapentin (p<0.05); and at 6 hours post-treatment with 30 mg/kg gabapentin (p<0.05) and 60 mg/kg gabapentin (p<0.05). Data were analyzed using GraphPad Prism version 10.0.2

Method: Modified Irwin functional observational battery

A heat map depicts the severity of the behaviors observed by two reviewers in a modified Irwin test at baseline and at 1, 2, 4, and 6 hours following the administration of vehicle (saline, delivered PO) or gabapentin (30, 60, 100, or 300 mg/kg, delivered PO). There were 8 rats per group (4 male and 4 female). Severity score was calculated as the sum of scores given by the two reviewers for all animals at that dose and timepoint divided by the maximum possible score, transformed into a percentage. In summary, gabapentin administration produced effects including decreased body position, decreased locomotion, sedation, and increased pupil size at all doses.

Method: Modified Irwin functional observational battery

Two graphs show the body temperature of male or female rats during the modified Irwin test. Responses are shown at the following timepoints: baseline (before treatment) and at 1 and 6 hours after treatment with vehicle (saline, delivered PO) or gabapentin (30, 60, 100, or 300 mg/kg, delivered PO). There were 4 rats per group. A significant effect of time was found in a two-way repeated measures ANOVA for females, but no significant effects were found in a two-way repeated measures ANOVA for males. Dunnett’s tests found a significant decrease in body temperature at 6 hours post-treatment in females (p<0.0001) relative to baseline. Data were analyzed using GraphPad Prism version 10.0.2

Method: Rat Rotarod

Animals that achieved a latency of 40 seconds during the baseline trial were included in the study.

Two graphs show the latency for male or female rats to fall off a rotarod apparatus. Responses are shown at the following time points: baseline (before treatment) and at 1, 2, 4, and 6 hours after treatment with vehicle (saline, delivered PO) or gabapentin (30, 60, 100, or 300 mg/kg, delivered PO). There were 10 rats per group. Dunnett’s tests performed after finding a significant effect of time in a two-way repeated measures ANOVA for males found a significant decrease in the latency to fall relative to baseline at 1 hour (p<0.05) and 2 hours (p<0.05) after treatment. Females had a significant interaction of time x treatment in a two-way repeated measures ANOVA, but Bonferroni’s tests found no significant post hoc comparisons. Data were analyzed using GraphPad Prism version 10.0.2

Model: Plantar Incision Model in Rat (Brennan Model)

Animals were balanced across treatment groups based on body weight and post-surgery withdrawal thresholds.

Animals with ipsilateral post-surgery withdrawal thresholds ≤3.0g were included in the study.

Endpoint: Rat Hind Paw Mechanical Allodynia Behavior (von Frey Filaments) Method

Two graphs show ipsilateral hind paw mechanical allodynia behavior assessed with von Frey filaments in male or female rats that have undergone plantar incision surgery. Responses are shown at the following time points: baseline (before surgery), prior to treatment at 1 day after surgery, and at 1, 2, 4, and 6 hours after treatment at 1 day after surgery. The treatments are vehicle (saline, delivered PO), gabapentin (10, 30, 60, or 100 mg/kg, delivered PO), or morphine (6 mg/kg, delivered SC). There were 10 rats per group. A significant interaction of time x treatment was found in a two-way repeated measures ANOVA for each sex. Dunnett’s tests found significant increases in withdrawal thresholds relative to vehicle in males at 1 hour post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.01, 0.0001, 0.001, 0.0001, respectively) and morphine (p<0.0001); at 2 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.001, 0.0001, 0.0001, 0.0001, respectively) and morphine (p<0.0001); at 4 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.01, 0.001, 0.0001, 0.0001, respectively) and morphine (p<0.0001); and at 6 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.05, 0.05, 0.001, 0.001, respectively) and morphine (p<0.01). Dunnett’s tests found significant differences increases in withdrawal thresholds relative to vehicle in females at 1 hour post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.001, 0.001, 0.0001, 0.0001, respectively) and morphine (p<0.0001); at 2 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.01, 0.001, 0.0001, 0.0001, respectively) and morphine (p<0.0001); at 4 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.05, 0.01, 0.001, 0.0001, respectively) and morphine (p<0.0001); and at 6 hours post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.01, 0.01, 0.01, 0.001, respectively) and morphine (p<0.001). Data were analyzed using GraphPad Prism version 10.0.2

Model: Plantar Incision Model in Rat (Brennan Model)

Animals were balanced across treatment groups based on body weight and post-surgery guarding scores.

Animals with ipsilateral post-surgery guarding scores ≥10 were included in the study.

Endpoint: Guarding Behavior

Two graphs show ipsilateral hind paw guarding behavior evaluated in male or female rats that have undergone plantar incision surgery. Responses are shown at the following time points: baseline (before surgery), prior to treatment at 1 day after surgery, and at 1, 2, 4, and 6 hours after treatment at 1 day after surgery. The treatments are vehicle (saline, delivered PO), gabapentin (10, 30, 60, or 100 mg/kg, delivered PO), or morphine (6 mg/kg, delivered SC). There were 10 rats per group. A significant interaction of time x treatment was found in a two-way repeated measures ANOVA for each sex. Dunnett’s tests found significant decreases in guarding scores relative to vehicle in males at 1 hour post-treatment with 10, 30, 60, and 100 mg/kg gabapentin (p<0.05, 0.05, 0.01, 0.0001, respectively) and morphine (p<0.0001); at 2 hours post-treatment with 60 and 100 mg/kg gabapentin and morphine (p<0.0001 for all comparisons); at 4 hours post-treatment with 60 mg/kg gabapentin (p<0.001), 100 mg/kg gabapentin (p<0.0001), and morphine (p<0.0001); and at 6 hours post-treatment with 100 mg/kg gabapentin (p<0.0001) and morphine (p<0.01). Dunnett’s tests found significant decreases in guarding scores relative to vehicle in females at 1 hour post-treatment with 100 mg/kg gabapentin (p<0.01) and morphine (p<0.0001); at 2 hours post-treatment with 60 mg/kg gabapentin (p<0.01), 100 mg/kg gabapentin (p<0.001), and morphine (p<0.01); and at 4 hours post-treatment with 60 mg/kg gabapentin (p<0.01) and 100 mg/kg gabapentin (p<0.05). Data were analyzed using GraphPad Prism version 10.0.2

Model: L5/L6 Spinal Nerve Ligation Model

Endpoint: Rat Hind Paw Mechanical Allodynia Behavior (von Frey Filaments) Method

Model: L5/L6 Spinal Nerve Ligation Model

Animals were balanced across treatment groups based on body weight and post-surgery positive responses to acetone trials.

Animals with ipsilateral post-surgery number of positive responses to acetone trials ≥2 were included in the study.

Endpoint: Rat Hind Paw Cold Allodynia (Acetone Evaporation) Test

Two graphs show ipsilateral hind paw cold allodynia behavior assessed with 5 trials of the acetone evaporation test in male or female rats that have undergone L5/L6 spinal nerve ligation surgery. Responses are shown at the following time points: baseline (before surgery), prior to treatment at 3 weeks after surgery, and at 1 and 3 hours after treatment at 3 weeks after surgery. The treatments are vehicle (saline, delivered PO), gabapentin (10, 30, 60, or 100 mg/kg, delivered PO), or the positive control morphine (6 mg/kg, delivered SC). There were 10 rats per group. A significant interaction of time x treatment was found in a two-way repeated measures ANOVA for each sex. Dunnett’s tests found significant decreases in acetone responses relative to vehicle in males at 1 hour post-treatment with 100 mg/kg gabapentin (p<0.05) and morphine (p<0.0001) and at 3 hours post-treatment with 60 mg/kg gabapentin (p<0.001), 100 mg/kg gabapentin (p<0.001), and morphine (p<0.001). Dunnett’s tests found significant decreases in acetone responses relative to vehicle in females at 1 hour post-treatment with 100 mg/kg gabapentin (p<0.01) and morphine (p<0.0001) and at 3 hours post-treatment with 100 mg/kg gabapentin (p<0.01). Data were analyzed using GraphPad Prism version 10.1.2

This work was conducted by PsychoGenics Inc. (Paramus, NJ) in collaboration with PSPP, NINDS, NIH under contract # 75N95019D00026. Prescribing information for clinically used controls can be found at labels.fda.gov. Information for icons representing experimental design details can be found through the NINDS Office of Research Quality https://go.nih.gov/Yw2tHGI.

Gabapentin

Formulation

Results

Protein Binding

Pharmacokinetics: Concentration in Plasma and Brain

Pharmacokinetics: Calculated Parameters

Irwin: Total Observations

Irwin: Behavioral Severity Scores

Irwin: Body Temperature

Rotarod

Plantar Incision: von Frey

Plantar Incision: Guarding Behavior

L5/L6 Spinal Nerve Ligation: von Frey

L5/L6 Spinal Nerve Ligation: Acetone