Table 1.

In situ gelling systems used for ophthalmic drug delivery.

Material	Drug model	Animal model	In vivo studies	In vivo results	Ref
Thermo-responsive gelling systems
Poloxamer®	Loteprednol	Rabbit	Determination of drug concentration of aqueous humor by HPLC	AUC(0–10h) and C_max values was found 2.55-fold and 4.34-fold higher, respectively, for in situ gel compared with marketed formulation.	[19]
	MethazolamideRabbit	Rabbit	Determination of drug concentration of aqueous humor by HPLC. Measurement of intraocular pressure by indentation tonometer.	AUC(0–12h) was found 1.58-fold higher for in situ gel compared with Azopt®. No significant difference in the IOP lowering effect was found between in situ gel and Azopt®.	[20]
	Timolol	Rabbit	Biocompatibility study (slit lamp test and histopathology study). Determination of drug concentration of aqueous humor by HPLC. Measurement of intraocular pressure by indentation tonometer.	Good biocompatibility and no sign of irritation. AUC(0–240min) and C_max was found 1.1-fold higher and 1.33-fold lower, respectively, for in situ gel compared with standard eye drop. No significant difference in the IOP lowering effect was found between in situ gel and standard eye drop.	[21]
Poloxamer®-HPMC or Poloxamer®-HEC	Ciprofloxacin HCl	Rabbit	Assessment of antimicrobial efficacy by scoring system	Significant improvement of scoring for Poloxamer-HPMC and poloxamer-HEC in situ gels compared with Ciprofloxacin®.	[22]
PNIPAAm	Epinephrine	Rabbit	Measurement of intraocular pressure by ophthalmic tonometer.	In situ gel decreased IOP for 24h with a minimum of 8.9 mmHg at 4h. Standard eye drop decreased IOP for 6–7h with a minimum of 7.2 mmHg at 2h.	[23]
PNIPAAm-HA	Ketoconazole	Rabbit	Eye irritation test (Draize test). Assessment of antimicrobial efficacy by scoring system.	No sign of ocular irritation. 91.7% and 66.7% of eyes were cured with in situ gel and commercial eye drops, respectively.	[24]
	Cyclosporine A	Rabbit	Eye irritation test (Draize test). Determination of drug concentration in ocular tissues by HPLC.	No sign of ocular irritation. Significant increase of drug concentration levels in corneas (1455.8 ng/g of tissue) compared with castor oil formulation and commercial eye drops.	[25]
Xyloglucan	Pilocarpine	Rabbit	Assessment of pupil diameter.	AUC(0–270min) values were found higher for in situ gel compared with standard solution.	[26]
Glycerol 2-phosphatechitosan-gelatin	Levocetirizine	Rabbit and guinea pig	Eye irritation test. Precorneal drainage assessment by slit lamps and blue light. Assessment of antiallergic conjunctivitis efficacy by Evans Blue(EB) extravastion quantification.	No sign of ocular irritation. Residence time was found 2.94-fold higher for in situ gel compared with aqueous solution. Extravasted amounts of EB in ocular tissues were found 1.75-fold and 2.56-fold lower for aqueous solution and in situ gel, compared with physiological saline.	[27]
	Timolol	Rabbit	Eye irritation test. Precorneal retention time by fluorescein staining. Measurement of intraocular pressure by tonometer.	No sign of ocular irritation. Precorneal retention was around 10 min for standard eye drops and at least 60 min for in situ gel. The maximum IOP lowering effect was observed at 0.5h and 1h for standard eye drops and in situ gel, respectively. The IOP lowering effect lasted 12h and 24h for standard eye drops and in situ gel, respectively.	[28]
	Latanoprost	Rabbit	Measurement of intraocular pressure by tonometer.	Weekly administration of in situ gel showed similar IOP lowering effect pattern compared with daily administration of Xalatan®.	[29]
pH-responsive gelling systems
Carbopol®-HPMC	Puerarin	Rabbit	Determination of drug concentration of aqueous humor by HPLC.	AUC(0–24h) and C_max values were found 2.17-fold and 1.29-fold higher, respectively, for in situ gel compared with aqueous solution.	[30]
	Baicalin	Rabbit	Eye irritation test (Draize test). Determination of drug concentration in ocular tissues by HPLC.	AUC and C_max values were found 6.1-fold and 3.6-fold higher, respectively, for in situ gel compared with aqueous solution.	[31]
	Pefloxacin	Rabbit	Determination of drug concentration in ocular tissues by HPLC.	Drug concentration was found above MIC (minimum inhibitory concentration, 2 ng/mL) for 24h for in situ gel and for 12h for marketed eye drops.	[32]
	Timolol and brimonidine	Rabbit	Eye irritation test. Measurement of intraocular pressure by Schiotz tonometer.	No sign of ocular irritation. Maximum ΔIOP (IOP treated eye – IOP untreated eye) achieved 17.75±0.050 mmHg at 12h and 13.12±0.034 mmHg at 4h for in situ gel and COMBIGEN®, respectively. 14h after instillation, ΔIOP was found 2.51-fold higher for in situ gel compared with COMBIGEN®.	[33]
Carbopol®-Chitosan	Timolol	Rabbit	Measurement of intraocular pressure by Schiotz tonometer.	AUC(0–9h) values were found 1.71-fold and 2.48-fold higher for Carbomer® in situ gel and Carbomer®-Chitosan in situ gel, respectively, compared with GLUCOMOL®.	[34]
Ion-responsive gelling systems
Gellan gum	Moxifloxacin	Rabbit	Determination of drug concentration in ocular tissues by HPLC. Bacterial infection study.	AUC(0–∞) and C_max values were found 6-fold higher for in situ gel compared with Vigamox®. In situ gel cured corneal infection after 4 days compared to 7 days of photodynamic therapy.	[35]
	Brinzolamide	Rabbit	Eye irritation test (Draize test). Measurement of intraocular pressure by tonometer.	1h after instillation, IOP was found 18.2% and 27% lower for in situ gel and standard solution, respectively. After 6h, IOP was found lower for in situ gel (18.6 mmHg) compared to standard solution (21.2 mmHg).	[18]
Gellan gum-NaCMC	Gatifloxacin	Rabbit	Eye irritation test (Draize test). Assessment of antimicrobial efficacy on a S. aureus infection model by clinical symptoms scoring.	No sign of ocular irritation. Significant improvement in the observed symptoms for in situ gel compared with marketed solution.	[36]
Gellan gum-Kcarrageenan	Econazole	Rat	Eye irritation test (HET-CAM). Biopermanence PET study	No sign of ocular irritation. AUC(0–∞) was found 2.33-fold higher for in situ gel compared to standard solution.	[37]
Alginate-HPMC	Gatofloxacin	Rabbit	Eye irritation test (Draize test). Precorneal drainage assessment by gamma scintigraphy.	No sign of ocular irritation. AUC(0–10h) values were found 3.58-fold higher for in situ gel compared with standard eye drop.	[38]
Alginate-NaCMC	Gatifloxain	Rabbit	Eye irritation test (Draize test). Assessment of antimicrobial efficacy on a S. aureus infection model by clinical symptoms scoring.	No sign of ocular irritation. Significant improvement in the observed symptoms for in situ gel compared with marketed solution.	[36]
Alginate-Gellan gum	Matrine	Rabbit	Eye irritation test (Draize test). Determination of drug concentration in tear fluid by HPLC.	AUC(0–30) values were found 4.65-fold, 3.44-fold and 2.83-fold higher for alginate-gellan gum, alginate and gellan gum in situ gels respectively, compared to standard drug solution.	[39]
Multi-stimuli responsive gelling systems
Alginate-Poloxamer®	Pilocarpine	Rabbit	Assessment of pupil diameter.	AUC(0–360h) values were found 4.38-fold, 2.85-fold and 1.36-fold higher for alginate-poloxamer, poloxamer and alginate in situ gels, respectively, compared with standard solution.	[40]
Carbomer®-xanthan gum	Ofloxacin	Rabbit	Eye irritation test (Draize test). Determination of drug concentration in tear fluid by HPLC.	No sign of ocular irritation. In situ gel and Oflox® showed significant difference in residence time at all point intervals.	[41]
Alginate-chitosan	Levofloxacin	Rabbit	Eye inflammation with infra-red camera. Precorneal drainage assessment by gamma scintigraphy.	Standard eye drop cleared more rapidly from the corneal region and reached systemic circulation via nasolacrimal drainage, compared with in situ gel.	[42]
Chitosan-PNIPAAm	Timolol	Rabbit	Measurement of intraocular pressure by Schiotz tonometer.	At all time points, in situ gels exhibited stronger IOP lowering effect compared with standard solution. Maximum IOP decrease was found higher for in situ gel (3.375 kPa) compared with standard solution (2.395 kPa).	[43]

Abbreviations: AUC = Area under the curve; EB = Evans blue; HA = Hyaluronic acid; HCl = Hydrochloric acid; HEC = Hydroxyethyl cellulose; HET-CAM = Hen’s egg-chorioallantoic membrane test; HPLC = High performance liquid chromatography; HPMC = Hydroxypropyl methyl cellulose; IOP = Intraocular pressure; NaCMC = Sodium carboxymethycellulose; PET = Positron emission tomography; pNIPAAm = Poly(N-isopropylacrylamide); ΔIOP = Intraocular pressure variation