Rapamycin (also known as “sirolimus”) is a triene macrolide antibiotic produced by Streptomyces hygroscopicus, found in the soil on Easter Island, Chile in 1975. Rapamycin has an antifungal activity, particularly can against Candida albicans, both in vitro and in vivo. Rapamycin has a significant immunosuppressive activity which can be used for prophylaxis of allergic encephalomyelitis, multiple sclerosis and reactive arthritis. Rapamycin can also be used for prophylaxis and treatment of systemic lupus erythematosus, pneumonia, insulin-dependent diabetes mellitus, skin diseases (e.g. psoriasis), intestinal disorders, smooth muscle cell proliferation and vascular injury caused by intimal thickening, adult T-cell leukemia/lymphoma, ophthalmia, malignant cancer, inflammatory heart disease and anemia, etc. Rapamycin was developed by American Wyeth Company, and approved the listing for immunotherapy of patients after kidney transplantation by U.S. Food and Drug Administration (FDA) in 1999.
Rapamycin is the earliest mTOR (mammalian target of rapamycin) inhibitor (IC50=1.7 μmol/L) discovered, with a half-life of 40˜50 h in human body. In 1999 the FDA approved rapamycin as an immunosuppressive agent for kidney transplantation. In 2003 rapamycin was approved for drug-eluting stents by the FDA due to its anti coronary artery restenosis effect. In many tumor tissues and animal models, such as leukemia, breast cancer, pancreatic cancer, melanoma, small cell lung cancer, liver cancer, etc., rapamycin could concentration-dependently inhibit tumor cell growth.

Although rapamycin shows good prospects in clinical application, it still has a low bioavailability (<15%), poor water-solubility and other defects, a number of rapamycin derivatives with a high efficiency and specificity such as everolimus (2), temsirolimus (3), ridaforolimus (4) and the like were further developed in the late 1990s.
Everolimus (RAD001,2), that is 42-O-(2-hydroxyethyl)-rapamycin, is a novel oral mTOR inhibitor, with a better water-solubility than that of rapamycin, however, experiments have shown that everolimus had a poor oral bioavailability (about 15% to 30%), and half-life of 16˜19 h in human body. Temsirolimus (CCI-779,3) is obtained after the hydroxyl group on the 42th position of rapamycin being esterifed with 2,2-bis(hydroxymethyl) propionic acid, temsirolimus is suitable for intravenous administration, and it is hydrolyzed to be rapamycin in vivo with a half-life of 13˜15 h.
Ridaforolimus (deforolimus, AP23573, MK-8668, 4) is a semi-synthetic derivative designed by CADD with an inhibition activity against mTOR.
In addition to rapamycin derivatives have been listed and in clinical research stage, according to principle of prodrug, many studies of structure modification for rapamycin by small molecular groups are conducted, such as U.S. Pat. No. 6,342,507, US20050026868, US20050101624, U.S. Pat. No. 5,432,183, etc., a hydroxyl group, alkyl group, amino group or phosphoric acid group, etc., is introduced into rapamycin and its derivatives to achieve of purpose of increasing the water-solubility or enhancing the stability.
Polyethylene glycol (PEG) is a neutral polymer with a linear type or branched chains and various molecular weights and can be dissolved in water and majority of organic solvents. PEG is a viscous colorless liquid when its molecular weight is less than 1000; and it is a ceraceous white solid with a higher molecular weight, and the melting point of the solid is raised with increasing molecular weight and no longer increases until reaching 67° C. PEG is nontoxic in vivo and it is a pharmaceutical polymer material accepted by the FDA and collected in Chinese, British, American and other national pharmacopoeia. It is reported that PEGylation of organic molecules can increase the water-solubility thereof and impart other beneficial properties, such as improved half-life in plasma, improved biological distribution and reduced toxicity.
In U.S. Pat. Nos. 5,955,457, 5,780,462, 6,432,973, 6,331,547 and International Patent WO2007/103348, the preparation of conjugates of PEG with rapamycin and its derivatives is described. PEG or a mercapto derivative thereof is used to be linded with modified rapamycin or a derivative thereof to form a water-soluble derivative of rapamycin. Although this modification method could improve the water-solubility of rapamycin, a low loading rate for drug is obtained because each terminal group of polyethylene glycol molecule is bonded with only one drug molecule which causes great pressure on pharmaceutical preparations.
In Chinese Patent CN 02106691, CN 03801105, CN 200410048016 and CN200610150011, a method for preparation of a prodrug derivative by bonding polyethylene glycol with drug molecule through a cactus oligopeptide is described, wherein the drug mentioned include paclitaxel which is a terpenes compound and camptothecin which is an alkaloids compound, etc. Because of the two or more carboxyl groups in cactus oligopeptide which can bond with two or more drug molecules, therefore this method can greatly improve the loading rate of drug molecules on polyethylene glycol molecule. However, because of a large molecular structure of rapamycin and two reactive hydroxyl groups in the molecular structure, when the esterification reaction of rapamycin with polyethylene glycol-cactus oligopeptide is conducted, an incomplete reaction is easily obtained and a phenomenon of a rapamycin bonding with a plurality of polyethylene glycol molecules may appear, thus resulting in a reduced loading rate and complex reaction products, meanwhile the references described above do not disclose a group of —CO—CH2— as the linking group between PEG molecule and oligopeptide.
In the present invention previous experiment methods are improved: (1) first conducting an esterification connection of a small molecule fragment containing a carboxyl group and a latent amino group with rapamycin, and the reaction product is a small molecule compound which can be purified by conventional chemical methods such as separation on columns; (2) then converting the latent amino group into an amino group to give an amino acid ester of rapamycin by reduction, hydrolysis and the like methods; (3) finally conducting an amidation connection of the amino acid ester of rapamycin with polyethylene glycol-cactus oligopeptide, since the nucleophilicity of an amino group is much stronger than that of an alcoholic hydroxyl group, so the amidation reaction can be carried out more easily and more completely than the esterification reaction, and remaining hydroxyl groups of rapamycin are substantially non-reactive due to a much weaker activity thereof than that of an amino group. The improved method can not only improve the loading rate on polyethylene glycol, but also obtain a rapamycin derivative with a comparatively single structure.