Introduction

Pidotimod, (R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, is a chemical synthesized immunization accelerator, which was approved in Italy for use in the body to enhance immunity in 1993. Pidotimod stimulates innate immunity by promoting dendritic cell maturation and natural killer cell activation, increasing phagocytosis and chemotaxis, and upregulating Toll-like receptors. Pidotimod also stimulates adaptive immunity by upregulating the type 1 immune response and, conversely, downregulating type 2 immunity, promoting the production of secretory IgA, IgM, and IgG, and decreasing IgE production. It is indicated for use in adults and children over 3 years of age as immunostimulant therapy in patients with documented cell-mediated immunosuppression during respiratory and urinary tract infections[1].

Pidotimod has been used across a wide range of doses and many studies have used different dosages, even higher than the approved indications. Namely, in children, the recommended dosage for treating acute respiratory infections is 400 mg twice/daily for 15-20 days as an add-on to standard therapy. The proposed dose for preventing respiratory infections is 400 mg once a day for 60 days. In adults, the suggested dose for treating acute respiratory infections is 800 mg twice daily for 8 days in combination with antibiotics, and for prophylaxis against acute exacerbations, 800 mg once daily for up to 60 days. No dosage adjustments are needed for older people. Because food interferes with the absorption of the product, Pidotimod should be administered 2 hours before or after meals[2]. As the only biologically active immunomodulator that can be used orally, China has imported pidotimod since 2003 and gradually applied it to clinical practice [3].

Due to the existing synthetic routes to produce large amount of waste liquid, the high costs restricted the industrial development of Pidotimod. Therefore, the study of new synthetic routes has potential application value and practical application prospects. Based on the existing literature, Wang et al. designed a route to synthesize Pidotimod with L-cysteine as a raw material by cyclization, esterification, neutralization, condensation, hydrolysation and acidification, the yield was 55.1% and the purity was 99.6%[4].

Synthesis route of Pidotimod

The synthetic route of pidotimod is shown in Figure 1.

Results and Discussion

Cyclization reaction occurred, with relatively inexpensive L-cysteine as raw material, in the aqueous solution system. Cyclization to L-thiazolidine-4-carboxylic acid with theformaldehyde, the simple filtering to get the product, and the reaction conditions were optimized. Ultimately, selected reaction time was 3 h, the feed ratio of the raw material is 1:1.5. Recovered formaldehyde aqueous solution without any treatment can be applied twice, the optimized yield was increased from 84.1% to 92.0% and purity was 99.2%.

Esterification reaction occurred, using of ethanol and dichlorosulfoxide instead of the hydrochloric methanol or ethanol as an esterification reagent in the original literature[5]. The yield increased from the original 85% to 93%, the purity of 99.2%, while reducing the dosage of ethanol, the discharge of waste water, after optimization of the reaction conditions, the mass ratio of L-thiazolidine-4-carboxylic acid to ethanol was determined to be 1:7, the molar ratio of L-thiazolidine-4-carboxylic acid to thionyl chloride was 1:1.1, reflow time is 4 h, recycled ethanol can be applied directly twice without treatment, improved product yield and reduce production costs.

Condensation reaction occurred. The esterification product neutralized by NaHCO₃ was extracted by dichloromethane, and directly condensed with L-pyroglutamic acid under the action of dehydrating agent DCC without distillation separation, and then hydrolyzed to obtain the final product pidotimod. After optimizing the reaction conditions, the condensation temperature was finally determined to be the gradient heating method. That is to say, holding at 0-5°C for 1h, heating to 25-30°C for 10h. The formation of by-products was reduced by gradient heating method, and the yield of the product after recrystallization with water reached 64.5%, and the purity of the product was 99.6%.

Conclusions

Compared with the existing synthetic route, the new process of raw material prices low, simple process, easy operation and less waste liquid treatment, is conducive to industrial production. According to the data obtained from the laboratory , the amplification experiment was carried out. Through experimental verification, the final product synthesized by this route had good color properties, the purity was 99.7 %, and the total yield was 55.6 %, which could be industrialized.

References

[1]Marseglia GL, Gelardi M, Santus P, Ciprandi G. Reappraisal of Pidotimod: an immunomodulatory agent with 30-year evidence. Minerva Med. 2024;115(4):503-515.

[2]Mahashur a, Thomas PK, Mehta P, Nivangune K, Muchhala S, Jain r. Pidotimod: in-depth review of current evidence. Lung india 2019;36:422–33.

[3] Coppi G, Silingardi S. Pharmacokinetics of Pidotimod in Rats and Dogs[J]. Arzneimittel Forschung, 1994;44(12A): 1460-1464.

[4] Wang B. Study on the synthesis process of pidotimod [D]. Jinan University, 2017;6:1-67.

[5]Wang X, Zhou L,Yang B. An improved preparation method of pidotimod [J].Pharmaceutical research, 2012;31(7):388-389.