Propylene glycol

Soya lecthin, cholesterol, propylene glycol, sodium hydroxide and hydrochloric acid were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Anhydrous ethanol and methanol were supplied by Tianjin Kaitong Chemical Reagent Factory (Tianjin, China). PBS, phosphate-buffered solution, was supplied by Beijing Sora Biotechnology Co., Ltd. (Beijing, China). Potassium bromide was purchased from Shanghai Macklin Biochemical Co., Ltd. (99.5% of purity, Shanghai, China). Phloretin, propylene glycol, 1,1-Diphenyl-2-picryl hydrazyl radical (DPPH), 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic) (ABTS), salicylic acid, ferrous sulfate heptahydrate, hydrogen peroxide (30%) and potassium persulfate were purchased from Aladdin Biotechnology Co., Ltd. (Shanghai, China). All other reagents were of analytical grade, and all experiments used Milli-Q water.

The lekethromycin standard (batch no. D20170101, purity ≥97%) was obtained from Henan Pulike Biological Engineering Co., Ltd (Luoyang, Henan, China). An authentic reference standard of gamithromycin (CAS 145435-72-9, purity ≥97%, internal standard, IS) was purchased from Sigma-Aldrich (St. Louis, MO, USA). LC-MS grade methanol (MeOH), acetonitrile (ACN) and formic acid (FA) (98% purity) were supplied by Fisher Scientific (Pittsburgh, PA, USA). Propylene glycol, PEG400 and dimethyl sulfoxide (DMSO) were of analytical grade and were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing, China). Ultrapure water was obtained from a Millipore Milli-Q purification system (Bedford, MA, USA). Blank rat plasma with EDTA-k2 anticoagulant was centrifuged at 4000× g for 10 min and was stored at −80 °C until analysis.

Hydroxyethyl cellulose (3400–5000 mPa.s, 25°C), azone, glycerol, triethanolamine, and ethyl p-hydroxybenzoate were purchased from Aladdin-Reagent Co. Ltd. (Shanghai, China). Centella asiatica total glycosides (47.95%), asiaticoside standard (≥98% pure), and madecassoside standard were obtained from Pusi Biotechnology Co., Ltd. (Chengdu, China). β-Cyclodextrin, propylene glycol, sodium hydroxide, polyethylene glycol, and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acetonitrile was purchased from Tedia (Tedia, Fairfield, OH, USA). 2,4-Dinitrochlorobenzene was purchased from Huamai Technology Co., Ltd. (Wuhan, China). Recombinant human epidermal growth factor solution was purchased from Shenzhen Watsin Genetech Ltd. Dibasic sodium phosphate was purchased from Chengdu Jinshan Chemical Reagent Co., Ltd. Potassium dihydrogen phosphate was purchased from Sangon Biotech (Shanghai) Co., Ltd. Nutrient agar and eosin methylene blue were purchased from Beijing Solarbio Science and Technology Co., Ltd. Phosphoric acid, methanol, n-butanol, acetic anhydride, sulfuric acid, formaldehyde solution, sodium chloride, sodium citrate, and citric acid were purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). The diabetes-inducing agent streptozotocin (STZ) (S0130) was purchased from Sigma-Aldrich (Sigma-Aldrich, Shanghai, China).