In 2009, prostate adenocarcinoma was diagnosed in an estimated 192,280 men and resulted in an estimated 27,370 deaths in the United States [1
]. Although androgen deprivation therapy is effective for advanced or recurrent disease, nearly all patients will eventually develop castrate-resistant prostate cancer (CRPC) after a variable duration of response [2
]. The regimen of docetaxel and prednisone was approved by the United States Food and Drug Administration in 2004 based on a modest improvement (about 2.5 months) in overall survival over mitoxantrone [4
] and is the standard of care for first-line treatment of CRPC. However, the duration of response remains short with a median overall survival of approximately 18 months. Currently there is no standard second-line therapy for these men after progression on a taxane-based regimen. Mitoxantrone with corticosteroids was approved for palliative benefit only (i.e. symptom palliation, without proven survival advantage) in 1996 [5
]. Cabazitaxel with prednisone recently showed a statistically significant improvement in overall survival when compared against mitoxantrone with prednisone in men with CRPC previously treated with docetaxel [7
]; however, this regimen is not yet FDA-approved and there are concerns about the toxicity profile of cabazitaxel in this population. While some men with taxane-refractory CRPC may currently go on to receive mitoxantrone, it is clear that better treatments for this population are urgently needed.
As in many solid tumors, angiogenesis appears to have an important role in prostate cancer progression. Increased microvessel density in clinically localized prostate cancer is an independent prognostic factor for progression and survival [8
] and is associated with higher stage after radical prostatectomy as well as shorter time to recurrence after radiation therapy [10
]. Plasma levels of vascular endothelial growth factor (VEGF), a potent and specific stimulator of endothelial cell proliferation and angiogenesis [13
], are increased in patients with metastatic prostate cancer, both in comparison to patients with localized prostate cancer and normal controls [14
]. VEGF level has been demonstrated to be an independent prognostic factor in men with metastatic CRPC [15
]. Therefore, inhibition of angiogenesis may be a viable strategy for the treatment of CRPC.
2-methoxyestradiol (2ME2) is a naturally occurring estrogen metabolite with both anti-angiogenic and anti-proliferative activity. 2ME2 binds poorly (0.05% binding) to the estrogen receptor [16
] and has not shown estrogenic activity in model systems. Preclinical data support anti-tumor activity of 2ME2 in multiple prostate cancer cell lines, including androgen-independent lines [17
]. The antiangiogenic activity of 2ME2 has been demonstrated in vivo
in corneal micropocket [18
], chick chorioallantoic model systems [19
], and Matrigel plug assays [20
], as well as by the observation of reduced tumor vasculature in 2ME2-treated mice [21
]. 2ME2 appears to inhibit proliferation through induction of apoptosis by activation of p53 [22
] and inhibition of hypoxia-inducible factor 1 (HIF-1), an important transcription factor for angiogenesis [23
]. In addition, 2ME2 binds to tubulin, inducing mitotic arrest by suppression of microtubule dynamics [24
]. The tubulin interaction occurs upstream of the inhibition of HIF-1, providing a mechanistic link between the disruption of the microtubule cytoskeleton and inhibition of angiogenesis [25
]. This promising preclinical data supporting antitumor activity of 2ME2 led to further testing in the clinic.
In initial clinical studies, 2ME2 was formulated as a capsule; however, although 2ME2 capsules appeared to be safe, pharmacokinetic results showed that 2ME2 capsules did not achieve sufficient sustained plasma levels of 2ME2 in order to adequately evaluate its therapeutic potential. A phase II study of 2ME2 capsules in chemotherapy-naïve, castrate-resistant prostate cancer (CRPC) patients showed decreases in PSA velocity despite suboptimal exposures of 2ME2 [26
]. Therefore, 2ME2 was reformulated as a NanoCrystal®
colloidal dispersion (NCD), which demonstrated an improved pharmacokinetic (PK) profile and antitumor activity in preclinical studies. In animal models, anti-cancer activity was enhanced when there was relatively constant plasma exposure to 2ME2 (i.e. when delivered by implanted osmotic pumps or multiple daily oral doses) and a target minimum effective concentration of 3.3 ng/mL was identified [27
]. The recommended phase II dose of 2ME2 NCD is 1500 mg by mouth four times daily, as determined in two independent phase Ib studies [28
]. This dose of 2ME2 NCD was tested in the present study of patients with docetaxel-refractory metastatic CRPC.
Assessing response in metastatic CRPC is difficult due to the disease manifesting primarily as bony metastases, which led to a search for legitimate biomarkers of response. Prostate-specific antigen (PSA) response has been an acceptable surrogate with conventional cytotoxic chemotherapy [31
]; however, with novel agents like 2ME2 that are presumably cytostatic, PSA response might not accurately predict clinical benefit. Although paired tumor biopsies (pre- and post-treatment) have provided the primary source for examination of surrogate markers of antiangiogenic therapies, biopsies of osseous metastases in CRPC are invasive and not clinically practical. Fluorodeoxyglucose positron emission tomography (FDG-PET) scanning is a non-invasive, widely available, and technically consistent means to assess the activity of new agents in patients on phase II trials by imaging early tumor-related changes [32
]. Although early studies of FDG-PET in prostate cancer suggested a much lower sensitivity for bone metastases than reference bone scans, many of these studies were performed in patients at heterogeneous stages of disease [33
]. Concordance of FDG-PET with bone scan findings and PSA changes were much higher, with a greater potential of PET over bone scan in discriminating active lesions from quiescent disease, when studied in more uniform patient populations, such as those with androgen-independent disease [34
]. A more recent study, in patients with metastatic CRPC undergoing anti-microtubule cytotoxic chemotherapy, revealed a greater than 90% accuracy of FDG-PET in correctly identifying the patients’ response to treatment at 4 and 12 weeks, based on comparison with standard outcomes with PSA measurements, bone scans and computed tomography (CT). The authors concluded that FDG-PET is promising outcome measure in metastatic CRPC for dichotomizing patients as progressors or nonprogressors [36
]. For these reasons, FDG-PET was evaluated as a biomarker in the present study.
We conducted a phase II trial of 2ME2 NCD in patients with taxane-refractory, metastatic castrate-resistant prostate cancer. The primary endpoint was progression-free survival (PFS) at 6 months. Secondary endpoints included assessment of PSA response (PCWG1 consensus criteria [31
]) and objective response rate by RECIST [37
], as well as safety evaluation. An exploratory endpoint was the metabolic response on FDG-PET imaging in a subset of patients with PET-positive metastases. Pharmacokinetics samples were collected to assess the steady-state levels of 2ME2 and its primary metabolite, 2ME1.