Infertility is a global issue, affecting approximately 48.5 million couples around the world [1,2]. Assisted reproductive technology (ART), notably in vitro fertilization (IVF) and embryo transfer (ET) (IVF-ET), offers advanced solution for overcoming infertility [3,4]. Progesterone (PRG) plays a particularly pivotal role in IVF treatment, as it helps maintain the stability of the endometrium, promotes embryo implantation, and supports pregnancy development [[5], [6], [7]]. For patients with luteal phase deficiency or other symptoms of PRG insufficiency, supplementation with PRG can aid in sustaining rates of clinical pregnancy and live birth [8,9].

PRG is a hydrophobic, steroid hormone [10]. Currently, commercially available PRG formulations include oral capsules, oil-based injections and vaginal gel. Oral administration is hindered by the low solubility and prominent hepatic first-pass metabolism of PRG, resulting in low bioavailability (approximately 5 %) and ultimately leading to poor therapeutic efficacy [11,12]. PRG injection, utilizing oil as the solvent, can induce strong irritation and easily form local nodules, which are difficult to eliminate [13,14]. Furthermore, clinical guidelines recommend daily administration of PRG oil solution for at least eight weeks in ART. Such long-term and high-frequency injections poses considerable inconvenience to patients. Crinone®, a vaginal PRG gel containing 8 % PRG, provides patients an alternative treatment option [15]. However, Crinone® inevitably results in leakage, bleeding, and interference with coitus [16]. Therefore, reducing the frequency of administration to improve patient compliance and maintaining plasma levels within the therapeutic window are crucial issue that must be addressed in clinical practice.

Despite attempts have been made to use nanocarriers such as microspheres, liposomes, and nanosuspensions to deliver PRG, their clinical application is still limited by several disadvantages, including low drug load and biocompatibility, complex and costly production processes, etc. [[17], [18], [19]]. Furthermore, although many studies have reported the pharmacokinetics of nanocarrier-mediated delivery of PRG, there were few studies focusing on their in vivo pharmacodynamics and functions. Hence, an ideal sustained delivery system for PRG that overcomes such obstacles still needs to be developed, requiring a thorough study of their therapeutic effects simultaneously.

In-situ forming implants (ISFIs) systems are among the most crucial technologies used for sustaining drug release. Compared to conventional formulations, ISFIs have several merits, such as reduced administration dosage and frequency, ensured long-lasting therapeutic effect, and loaded active pharmaceutical ingredients (API) with dose-limiting toxicity. Such advantages not only enhance patient compliance but also reduce overall healthcare expenses. FDA-approved slow-release ISFIs platforms encompass a range of medications, such as Sublocade® (buprenorphine), Atridox® (doxycycline), Eligard® (leuprolide acetate), and Sandostatin® (octreotide). As one of the ISFI technologies, phospholipid phase transition gels (PPTGs) have attracted considerable attention due to their numerous advantages including ease of preparation, less toxicity compared to poly (lactide-co-glycolide) (PLGA)-based ISFIs, excellent biocompatibility, and controlled and sustained drug release, making them a promising option for delivering PRG [[20], [21], [22]]. PPTGs typically consist of phospholipid with low toxicity and excellent biocompatibility, along with ethanol as a solvent [23,24]. They can be easily prepared through a simple stirring process and undergo an immediate phase transition upon exposed to water, transitioning in situ from solution to gel-like depot structure, thereby facilitating sustained drug release [25,26]. The in-situ phase transition is a critical phenomenon primarily driven by the solvent exchange mechanism, where the diffusion of the solvent into the adjacent tissues triggers the gel transition of phospholipid [27,28].

In this study, we first incorporated PRG into a phospholipid and cholesterol-based phase transition gel. Later, we found that the incorporation of glyceryl monostearate (GMS) as a surfactant not only reduced the concentration of ethanol in the PRG-PPTGs but also extended the release time of PRG. GMS is a solid lipid at ambient temperature, consisting primarily of mono- and di-acylglycerols. It is commonly utilized in the cosmetics, food, and pharmaceutical industries due to its excellent safety profile, being considered non-toxic and non-irritating [29]. Subsequently, we compared the therapeutic effects of different administration strategies on uterine development, and the results showed that PRG-PPTGs have the same effect in promoting uterine development as commercial PRG oil solution. However, PRG-PPTGs only need to be injected once a week, greatly reducing the frequency of administration and improving the convenience for patients. Furthermore, the subcutaneous injection route of the proposed formulation allows for self-administration, potentially increasing its acceptability and accessibility among users. Overall, our clinically feasible strategy provides a better way to supplement PRG in clinical practice through a simple and convenient method.