Preclinical studies have demonstrated that nab-paclitaxel may play a role in sensitizing the tumor to chemotherapeutic agents and specifically increases the antitumor efficacy when combined with gemcitabine. While the mechanism of action for the synergy is unclear, preclinical studies have generated hypothetical models. One hypothesis is a remodeling and weakening of the stroma barrier, allowing the chemotherapeutic agents to have better access to the tumor cells. Weakening the tumor-stroma barrier is particularly important in cancer that is characterized by dense stroma, such as pancreatic cancer. In mice with primary patient derived pancreatic tumor xenografts, nab-paclitaxel plus gemcitabine versus gemcitabine alone resulted in increased tumor regression and depleted the desmoplastic stroma as observed by the less dense, disorganized, wisps of collagen type1 fibers after 4 weeks of treatment. In this study, the intratumoral concentration of gemcitabine was increased by 2.8-fold after 5 days of treatment when nab-paclitaxel was added to gemcitabine. It was hypothesized that nab-paclitaxel may play a role in reducing the dense stroma and may have contributed to the increased intratumoral gemcitabine uptake. Additional preclinical studies in a genetically engineered mouse model of pancreatic adenocarcinoma, coadministration of nab-paclitaxel and gemcitabine also demonstrated tumor regression and increased intratumoral gemcitabine levels after 8 days of treatment. Apoptosis of tumor epithelial cells were observed; however, there were no changes in stromal components or collagen density in this short term treatment model. The increased intratumoral gemcitabine levels were attributed to a marked decrease in the primary gemcitabine metabolizing enzyme, cytidine deaminase, by nab-paclitaxel. Finally, a recent clinical study in subjects with resectable pancreatic cancer treated with neoadjuvant nab-paclitaxel plus gemcitabine showed reduction in fibrotic collagenous stroma, further supporting a stroma active mechanism for nab-paclitaxel.
In a clinical Phase 1/2 dose ranging study (CA040, NCT003980860), nab-paclitaxel plus gemcitabine (CA040, NCT003980860) antitumor activity and tolerability were established in patients who had no prior treatment for metastatic pancreatic cancer. The maximum tolerated dose and recommended dose for further studies was determined to be 125 mg/m2 nab-paclitaxel in combination with 1000 mg/m2 gemcitabine.
In the subsequent randomized international Phase 3 study (MPACT, CA046, NCT00394251) that enrolled 861 patients with metastatic pancreatic cancer, nab-paclitaxel in combination with gemcitabine exhibited a clinically meaningful, statistically significant improvement in OS and progression-free survival (PFS). The median OS (primary endpoint) in the intent-to-treat population was 8.5 months (95% CI = 7.89-9.53) with nab-paclitaxel/gemcitabine compared with 6.7 months (95 % CI = 6.01-7.23) with gemcitabine, p \< 0.0001, HR = 0.72 (95% CI = 0.617-0.835). Long-term survival was improved in the nab-paclitaxel/gemcitabine arm versus gemcitabine alone, with a 59% increase at 1 year (35% versus 22%) and doubling at 2 years (9% versus 4%). The secondary (PFS, overall response rate \[ORR\]) and all other efficacy endpoints showed consistent, statistically significant improvements with nab-paclitaxel/gemcitabine, supporting the results from the primary analysis of OS. Specifically, PFS (by independent review) was 5.5 months (95% CI = 4.47-5.95) versus 3.7 months (95% CI = 3.61-4.04) in the nab-paclitaxel/gemcitabine arm versus gemcitabine alone arms, respectively p \< 0.0001; HR =0 .69; 95% CI = 0.581-0.821). The improvement in PFS corresponded to a 31% reduction in the risk of progression or death with nab-paclitaxel/gemcitabine. Furthermore, in this study of metastatic unresectable adenocarcinoma of the pancreas, subjects in the combination arm were on therapy longer than those receiving single agent gemcitabine, indicating disease improvement and tolerable treatment. The suitability of the dosing regimen was confirmed by the observation that the majority of patients did not require a dose reduction, and that 71% of nab-paclitaxel doses were delivered at the starting dose of 125 mg/m2. The safety profile for both regimens was consistent with previous reports. Serious life threatening toxicities were not increased; AEs were acceptable and manageable. The most notable differences in toxicity between the 2 treatment arms was peripheral neuropathy, which was cumulative and rapidly reversible with dose delay and reduction, and neutropenia, which was manageable with dose delays and dose reductions. The incremental risks of sepsis and pneumonitis were managed by protocol amendments to increase awareness, and for early diagnosis and treatment to reduce the risk of fatal outcomes. Since the above described initial analysis of the MPACT study, the updated OS with a cutoff of May 2013 showed that the benefit continued to improve with nab-paclitaxel in combination with gemcitabine, with 8.7 versus 6.6 median months, respectively. The updated survival rates also significantly favored nab-paclitaxel plus gemcitabine at year 1 (35% versus 22%), year 2 (10% versus 5%), and year 3 (4% versus 0%) as compared with gemcitabine alone.
