Lower irritation microemulsion-based rotigotine gel: formulation optimization and in vitro and in vivo studies

Figures & data

Table 1 Weight ratios of Cremophor^® RH40, Labrasol^® and Transcutol^® HP and the corresponding pseudoternary phase diagram numberings

Ks (w/w)	Km (w/w)
	2:1	3:1	4:1
1:1	A	B	\
2:1	\	C	D
3:1	\	E	F

Note: A to F means the pseudoternary phase diagram shown in Figure 1.

Abbreviations: K_s, Cremophor RH 40:Labrasol (w/w); K_m, surfactants:cosurfactant (w/w); “\”, signifies there were no microemulsions formed at this proportion.

Figure 1 Pseudoternary phase diagram of the different S_mix systems containing the oil phase (Labrafil^®), surfactants (Labrasol^® and Cremophor^® RH40), cosurfactant (Transcutol^® HP), and aqueous phase (distilled water).

Note: The gray areas represent the microemulsion regions.

Abbreviations: K_m, weight ratio between surfactants and cosurfactant; K_s, weight ratio between two surfactants ie. Cremophor RH40 and Labrasol; S_mix, mixture of Cremophor RH40 Labrofil and Transcutol HP.

Table 2 Variables and observed responses in the CCD for the microemulsion

	Independent variables		Dependent variables		Viscosity (mPa · s)	Particle size (nm)	Polydispersity index
	X1	X2	Y1(Q24)	Y2(Jss)
ME1	0.65	0.119	58.22	3.3735	33±1.56	15.6±1.20	0.101±0.0023
ME2	0.65	0.225	132.91	6.3521	37±2.38	23.1±0.89	0.114±0.0051
ME3	0.509	0.225	91.21	4.2839	254±5.21	21.5±0.97	0.197±0.0039
ME4	0.75	0.3	80.32	3.7427	15±1.67	36.8±0.44	0.125±0.0060
ME5	0.75	0.15	49.43	2.8226	22±3.33	18.9±1.34	0.089±0.0031
ME6	0.65	0.225	129.19	6.2363	37±2.38	23.1±1.09	0.114±0.0051
ME7	0.55	0.3	89.23	4.1527	159±5.23	46.6±2.08	0.218±0.0012
ME8	0.791	0.225	68.86	3.2545	18±2.33	24.2±2.12	0.096±0.0027
ME9	0.65	0.225	131.88	6.3788	37±2.38	23.1±0.89	0.114±0.0051
ME10	0.65	0.225	127.65	6.2436	37±2.38	23.1±0.89	0.114±0.0051
ME11	0.65	0.331	103	3.9042	25±3.89	65.4±2.29	0.144±0.0042
ME12	0.55	0.15	60.4	3.4354	226±5.76	16.2±0.84	0.128±0.0045
ME13	0.65	0.225	135.43	6.5222	37±2.38	23.1±0.89	0.114±0.0051
Independent variables			Levels used, actual (coded)
			−1	0	+1	−alpha	+alpha
X1 = water/microemulsion (w/w)			0.55	0.65	0.75	0.509	0.791
X2 = oil/Smix (w/w)			0.15	0.225	0.3	0.119	0.331

Note: Each data represents the mean ± SD (n=3).

Abbreviations: CCD, Central composite design; J_ss, steady-state flux; ME, microemulsion formulation; Q₂₄, permeation of rotigotine in 24 hours; SD, standard deviation.

Table 3 Solubility of rotigotine in various solvents

Phase type	Solvent	Solubility (mg/mL)
Oil phase	Olive oil	10.25±0.16
	Labrafil^® M 1944 CS	47.22±0.27
	Labrafac^® Lipophile	46.32±0.34
	Oleic acid	16.16±0.13
Surfactant	Tween^®80	13.1±0.16
	Labrasol^®	87.47±0.32
	Cremophor^®RH40	21.36±0.26
Cosurfactant	Transcutol^® HP	259.85±0.68
	PEG-400	110.06±0.45
	1,2-propylene glycol	8.83±0.14
	Absolute ethyl alcohol	99.81±0.28

Note: Each data represents the mean ± SD (n=3).

Abbreviations: PEG, polyethylene glycol; SD, standard deviation.

Table 4 Analysis of variance for the responses Y₁ and Y₂ after fitting by the quadratic model

Quadratic model	F	P	R^2	Pred R^2	Adj R^2	AP	SD	C.V.%
Response (Y1)	60.79	<0.0001	0.9775	0.8570	0.9614	18.437	6.28	6.49
Response (Y2)	147.45	<0.0001	0.9906	0.9455	0.9839	27.404	0.18	3.89

Notes: R² is relevant index which reflect a fitting degree; Pred R² (predicted R²) and Adj R² (adjusted R²) are reasonably agree with each other represent a good model fitting; AP is “Adeq precision”, which measures the singal to noise ratio.

Abbreviations: AP, Adeq precision; C.V.%, coefficient of variation; SD, standard deviation.

Figure 2 Linear correlation plots (A and B) between the actual and predicted values.

Abbreviations: J_ss, steady-state flux; Q₂₄, permeation of rotigotine in 24 hours.

Figure 3 The contour plot (A and B) and corresponding surface response plots (C and D).

Abbreviations: J_ss, steady-state flux; Q₂₄, permeation of rotigotine in 24 hours; S_mix, mixture of Cremophor RH40, Labrofil and Transcutol HP.

Table 5 The predicted and experimental values of the response variables and predicted error percentages

Responses	Predicted value	Experimental value	Percentage prediction error (%)
Q24(μg/cm^2)	125.57	123.84±3.95	−1.46±3.23
Jss (μg/cm^2·h)	5.81	5.84±0.07	0.58±1.12

Abbreviations: J_ss, steady-state flux; Q₂₄, permeation of rotigotine in 24 hours.

Figure 4 In vitro permeation profiles of rotigotine in the microemulsions and oil (Labrafil^®).

Note: Each point represents the mean ± SD (n=3).

Abbreviations: ME, microemulsion formulation; SD, standard deviation.

Figure 5 Transmission electron microscopy of the rotigotine-loaded microemulsion.

Figure 6 Accumulated amount of rotigotine that permeated from the optimal formulation of the rotigotine microemulsion gels at different Carbomer concentrations (w/w).

Note: The values represent the mean ± SD (n=3).

Abbreviations: ME, microemulsion formulation; SD, standard deviation.

Table 6 Pharmacokinetic parameters of the rotigotine microemulsion gel and Neupro^® patch

Dosing form	t1/2 (h)	Tlag	Tmax (h)	Cmax (ng/mL)	AUC0→48h (μg·h/mL)	Frel (%)
Microemulsion gel	13.87±4.38	0	6.67±1.03	12.15±3.67	179.68±34.85	105.76±20.52
Neupro^®	22.35±12.83	0	7.33±2.42	9.83±2.80	169.39±30.77	100

Notes: Each data represents the mean ± SD (n=6).

Abbreviations: AUC, area under the concentration–time curve; C_max, maximum blood concentration; Frel, relative bioavailability; SD, standard deviation; t_½, half-life; T_lag, lag time; T_max, time to reach C_max.

Figure 7 Drug–time curves in rats administered the rotigotine microemulsion gel and Neupro^® patch.

Note: Each point represents the mean ± SD (n=6).

Abbreviation: SD, standard deviation.

Table 7 The average skin irritation response scores

Dosing form	Average score of erythema
Microemulsion gel	0
Neupro^®	1.33±0.52

Note: Each data represents the mean ± SD (n=6).

Abbreviation: SD, standard deviation.

Figure 8 Skin irritancy studies on rat abdomens.

Notes: (A) No drug. (B) Rotigotine microemulsion gel administration. (C) Neupro^® patch administration.