7-Ethyl-10-hydroxycamptothecin (SN38), the active metabolite of irinotecan, demonstrate a potency 100 to 1000-fold higher than irinotecan itself against various tumor cell lines. In vitro cytotoxicity assays reveal that the potency of SN-38, in comparison to irinotecan, ranges from 2 to 2000-fold. Nevertheless, certain unfavorable physical and chemical properties of this compound, such as poor solubility and stability, significantly impede its effecient delivery to tumor sites.

As the primary component of plasma proteins, albumins emerge as promising biomimetic materials with the capacity to transport both hydrophilic and hydrophobic molecules. Notably, they are reported to have binding sites for camptothecins on subdomain IB. In addition, the uptake of albumin nanoparticles is suggested though GP60/SPARC receptors, which are overexpressed in colon, prostate, pancreatic, and breast cancers, leading to an enhanced anticancer effect.

To enhance water solubility and antitumor efficacy, SN38 was covalently bonded to the only free sulfhydryl at cysteine-34 on the Bovine serum albumin (BSA). The conjugation is site specific and employs a thiol-binding linker to form a prodrug BSA-SN38 conjugate (BSA:SN38 = 1:1). The conjugate dramaticlly incresed the water solubility of the anticancer drug as the carrie protein BSA is water soluble. The high potent drug loading can be precisely controlled as the conjugation is site specific. The water-soluble BSA-SN38 conjugate has been reported to demonstrate potent antitumor effects against colorectal, lung, and ovarian cancers.

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Reference Reading:

1. Recent advances in SN-38 drug delivery system

J CH Yang, L R Jia, ZH G He, Y J Wang. International Journal of Pharmaceutics. 2023, 637, 122886

DNA topoisomerase I plays a key role in lubricating the wheels of DNA replication or RNA transcription through breaking and reconnecting DNA single-strand. It is widely known that camptothecin and its derivatives (CPTs) have inhibitory effects on topoisomerases I, and have obtained some clinical benefits in cancer treatment. The potent cytotoxicity makes 7-ethyl-10-hydroxycamptothecin (SN-38) become a brilliant star among these derivatives. However, some undesirable physical and chemical properties of this compound, including poor solubility and stability, seriously hinder its effective delivery to tumor sites. In recent years, strategies to alleviate these defects have aroused extensive research interest. By focusing on the loading mechanism, basic nanodrug delivery systems with SN-38 loaded, like nanoparticles, liposomes and micelles, are demonstrated here. Additionally, functionalized nanodrug delivery systems of SN-38 including prodrug and active targeted nanodrug delivery systems and delivery systems designed to overcome drug resistance are also reviewed. At last, challenges for future research in formulation development and clinical translation of SN-38 drug delivery system are discussed.

2. Synthesis, characterization, and antitumor evaluation of the albumin–SN38 conjugate

Y Q Yao, X L Sua, Y M Xie, Y X Wang, TR Kang, L T Gou, CH Yi, J L Yang. Anti-Cancer Drugs. 2013, 24:270–277

7-Ethyl-10-hydroxycamptothecin (SN38), the active metabolite of irinotecan, exerts a 100-fold to 1000-fold higher effect than irinotecan itself against several tumor cell lines. However, the water insolubility of SN38 has prevented its direct use as an antitumor drug in the clinic. To improve the water solubility and antitumor efficacy, SN38 was covalently attached to the only free sulfhydryl at cysteine-34 on the BSA site specifically through a thiol-binding linker to form a prodrug BSA-SN38 conjugate (BSA: SN38 = 1:1). The water solubility of this conjugate was similar to albumin using the current method. Also, SN38 loading in this conjugate became controllable. Size-exclusion chromatography purification and UV characterization of the SDS-PAGE electrophoresis product were carried out. Then, an MTT assay was carried out to test the antitumor effect of this conjugate on five colon cancer cell lines in vitro. The 72 h IC50 values of the BSA-SN38 conjugate ranged from 1.5 to 6.1 umol/l. A colorectal peritoneal carcinomatosis model in mice was established to determine the intraperitoneal chemotherapy effect of the BSA-SN38 conjugate. The BSA-SN38 conjugate at an SN38 equivalent dose of 10 mg/kg/day was administrated every 4 days. Eighteen days after manipulation, the mice were euthanized and the tumors in the abdominal cavity were collected and weighed. Tumors in the BSA-SN38 conjugate treatment group (m = 0.21±0.15 g) were found to be significantly (P = 5) lighter than those in the NS control group (m = 4.74±0.73 g). The results indicated that this water-soluble BSA-SN38 conjugate exerted a strong antitumor effect on colorectal carcinoma.

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