Background: Breast cancer is the most common malignancy among women, and has one of the highest fatality rates of all cancers affecting females.Taxanes are cornerstones in the treatment of breast cancer.However, paclitaxel is difficult to formulate for parenteral administration because of its low solubility and Cremophor, the excipient used for its Cremophor-based paclitaxel formulation, has been shown to cause serious side effects.So far, the most sucessfully taxane alternative to avoid Cremophor-associated toxicities was ABI-007, an albumin-bound, 130-nm particle form of paclitaxel.Recently, a novel cremophor-free, lipid microsphere loaded with lipid-complexed paclitaxel (lipo-paclitaxel) was developed in our laboratories.This lipid microsphere, with a particle size of 135 nm, had many favorable properties such as drug-carrier bioeompatibility, autoclave stability and ease of preparation.It also exhibited a better tolerance than an equal dose of Cremophor-based paclitaxel in guinea pigs, as no obvious hypersensitivity reaction was observed.Objective: The objective of the article was to investigate the potential efficacy of lipo-paclitaxel in the treatment of breast cancer.We studied the antitumor activity of lipo-paclitaxel in nude mice bearing subcutaneous, implanted MDA-MB-231 human mammary tumors.Methods: To obtain the equitoxic doses for the efficacy comparison of lipo-paclitaxel and its control drug Cremophor-based paclitaxel, a pre experiment for the determination of the maximum tolerated doses (MTDs) were performed in non-tumor-bearing nude mice.5 to 8 mice animals per dose group were given drugs once every 4 day for 3 consecutive cycles.The dose levels were designed at 30, 45, 67.5, and 101.25 mg/kg/d for lipo-paclitaxel, and 20, 30, and 45 mg/kg/d for Cremophor-based paclitaxel, respectively.Antitumor activity was performed in an 80-day study on MDA-MB-231 xenograft-tumored nude mice.8 or 9 mice per dose group were treated at the same therapy program for the determination of MTDs.According to the MTDs results, dose levels for the efficacy evaluation were designed at 20, 30, and 45 mg/kg/d for lipo-paclitaxel, and 20 mg/kg/d for its control drug Cremophor-based paclitaxel, respectively.At the same time, ABI-007 was adopted as the second control drug of lipo-paclitaxel and its dose levels were designed at 20, 30, and 45 mg/kg/d.The tumor size and animal weight were measured once every three days.Animals were sacrificed on day 32 for un-treated control group, 57 for treated groups at 20 mg/kg/d, 67 for treated groups at 30 mg/kg/d, and 80 for treated groups at 45 mg/kg/d, respectively.Results: The survivors of lipo-paclitaxel in non-tumor-bearing nude mice at 30, 45, 67.5, and 101.25mg/kg/d were 0 of 8, 1 of 8, 1 of 8, and 2 of 8, respectively, and Cremophor-based paclitaxel at 20, 30, and 45 mg/kg/d were 0 of 8, 6 of 8 (at the end of the second cycle), and 3 of 8 (at the end of the first cycle), respectively.Lipo-paclitaxel was significantly less toxic than Cremophor-based paclitaxel.MTDs were 45 or 67.5 mg/kg/d for lipo-paclitaxel, and 20 mg/kg/d for Cremophor-based paclitaxel.There was no obvious toxic response observed in lipo-paclitaxel-treated groups and ABI-007-treated groups on MDA-MB-231 xenograft-tumored nude mice model, compared with the significantly instant response in Cremophor-based paclitaxel-treated group.One mice receiving lipo-paclitaxel (20 mg/kg), lipo-paclitaxel (45 mg/kg), and ABI-007 (45 mg/kg) died on day 56, 57, 73, respectively, which might be related to bigger tumor load and longer observation time.Lipo-paclitaxel groups and ABI-007 groups at three dose levels did not show significantly different body weight from Cremophor-based paclitaxel (Fig.1).All the three formulations caused tumor regression and prolonged survival in MDA-MB-231 xenograft mice (Fig.2 and Fig.3).The doses had a significant correlation with tumor responses, and with the increased dose, both lipo-paclitaxel and ABI-007 exhibited an enhanced antitumor activity.Lipo-paclitaxel-treated groups at 30, and 45 mg/kg/d showed greater antitumor activity than Cremophor-based paclitaxel-treated groups at 20 mg/kg/d as measured by tumor volume, tumor recurrence time and tumor doubling time.Surprisingly, at equal dose level of 30, and 45 mg/kg/d, lipo-paclitaxel also exhibited better antitumor activity than ABI-007, including more complete regressions, longer time to recurrence, longer doubling time and prolonged survival.Conclusions: In summary, lipo-paclitaxel is a Cremphor-free, lipid-complexed paclitaxel lipid microsphere that showed an improved efficacy and therapeutic index in MDA-MB-231 human mammary tumor animal model.Lipo-paclitaxel is not only well tolerated but compared with ABI-007 and Cremophor-based paclitaxel, is associated with superior response rate, longer time to tumor recurrence, and prolonged survival.Therefore, this novel taxane is a good candidate for testing in clinical trials of breast cancer.