*Article* **New Biopharmaceutical Characteristics of In Situ Systems Based on Poloxamer 407**

**Elena O. Bakhrushina <sup>1</sup> , Elizaveta V. Novozhilova 1,2 , Marina M. Shumkova 1,\* , Victor S. Pyzhov <sup>1</sup> , Maria S. Nikonenko <sup>1</sup> , Alexander I. Bardakov <sup>1</sup> , Natalia B. Demina <sup>1</sup> , Ivan I. Krasnyuk, Jr. <sup>3</sup> and Ivan I. Krasnyuk <sup>1</sup>**


**Abstract:** Thermosensitive systems based on poloxamer 407 are widely used in targeted drug delivery; however, the stability of the phase transition temperature remains insufficiently studied. This article presents the results of a study on the effect of adding polyethylene glycols (PEG) with different molecular weights and some classical gel-forming polymers on the gelation temperature of thermoreversible compositions based on poloxamer 407 in a long-term experiment. The study showed a positive effect of PEG addition with average molecular weights at concentrations of 1.5–2.0%, as well as gelling agents at a concentration below the critical gelation concentration. The proposed rheological test for studying the samples' adhesion can give an indirect forecast of the composition adhesive rate. Based on the conducted studies, three experimental binary systems based on poloxamer 407 were selected, with the addition of HPMC 0.5%, sodium alginate 0.5%, and PEG 1500 1.5%. These systems are the most promising for the further development of in situ targeted drug delivery systems.

**Keywords:** in situ systems; Kolliphor; poloxamer; polyethylene glycol; gelation temperature; adhesion
