Table 1. Type I isobolographic analysis of interactions for collateral dose-response relationship lines between antiepileptic drugs (at the fixed drug dose ratio of 1:1) in the 6-Hz corneal stimulation-induced seizure model in mice.

Drug combination	ED50 exp (mg/kg)	n exp	ED50 add (mg/kg)	n add	I-index	Types of interaction
PGB + LCM	8.17 ± 2.40 *	24	18.11 ± 4.42	36	0.45	synergy
GBP + LEV	15.06 ± 5.03 **	16	43.27 ± 6.47	44	0.35	synergy
PGB + LEV	7.20 ± 2.39 **	16	23.04 ± 4.78	36	0.31	synergy
GBP + RTG	12.62 ± 3.81 ***	8	50.57 ± 7.30	52	0.25	synergy
LEV + RTG	3.24 ± 0.55 ***	16	21.73 ± 2.99	52	0.15	synergy
PGB + GBP	44.36 ± 5.51	16	51.88 ± 9.09	36	0.86	additivity
PGB + RTG	12.41 ± 3.32 **	16	30.34 ± 5.61	44	0.41	synergy

The experimentally-derived median effective doses (ED_{50 exp} values in mg/kg ± S.E.M.) for two-drug mixtures were statistically compared to their respective theoretically calculated additive median effective doses (ED_{50 add} values in mg/kg ± S.E.M.) by the use of unpaired Student’s t-test with Welch’s correction as recommended elsewhere [60, 86–88]. n _exp−total number of animals used at those doses whose expected antiseizure effects ranged between 16% and 84% (i.e., 4 and 6 probits) for the experimental mixture; n _add−total number of animals calculated for the additive mixture of the drugs examined (n _add = n_{_antiepileptic drug_1} + n_{_antiepileptic drug_2}−4); I-index–interaction index as a ratio of ED_{50 exp} and ED_{50 add} values

*P<0.05

**P<0.01

***P<0.001 vs. the respective ED_{50 add} value.