Pharmacological ascorbate with gemcitabine for the control of metastatic and node-positive pancreatic cancer (PACMAN): results from a phase I clinical trial

Abstract

Background: Treatment for pancreatic cancer with pharmacological ascorbate (ascorbic acid, vitamin C) decreases tumor progression in preclinical models. A phase I clinical trial was performed to establish safety and tolerability of pharmacological ascorbate combined with gemcitabine in patients with biopsy-proven stage IV pancreatic adenocarcinoma.

Design: Nine subjects received twice-weekly intravenous ascorbate (15-125 g) employing Simon's accelerated titration design to achieve a targeted post-infusion plasma level of ≥350 mg/dL (≥20 mM). Subjects received concurrent gemcitabine. Disease burden, weight, performance status, hematologic and metabolic laboratories, time to progression and overall survival were monitored.

Results: Mean plasma ascorbate trough levels were significantly higher than baseline (1.46 ± 0.02 vs. 0.78 ± 0.09 mg/dL, i.e., 83 vs. 44 μM, p < 0.001). Adverse events attributable to the drug combination were rare and included diarrhea (n = 4) and dry mouth (n = 6). Dose-limiting criteria were not met for this study. Mean survival of subjects completing at least two cycles (8 weeks) of therapy was 13 ± 2 months.

Conclusions: Data suggest pharmacologic ascorbate administered concurrently with gemcitabine is well tolerated. Initial data from this small sampling suggest some efficacy. Further studies powered to determine efficacy should be conducted.

Figure 1. Schematic of phase I clinical treatment plan

Red blood cell hemolysis may occur in people deficient in the G-6-PD enzyme, so this is an exclusion criterion. If the results of the G-6-PD laboratory test are negative, a test dose of 15 g of ascorbic acid was infused over 30 min. If tolerated throughout the first week, the dose of ascorbate was increased weekly (i.e., every 2 infusions) until the plasma level reaches at least 350 mg/dL (20 mM). Ascorbate infusions were given each week of the four-week cycle.

Figure 2. Ascorbate infusions achieve plasma millimolar levels

Dose escalation of ascorbate infusions was administered to achieve a target level of 350 – 450 mg/dL (20 – 25 mM). Once subjects achieved these levels, the dose of ascorbate was not changed. A. Peak ascorbate levels of 20 – 30 mM were reached with doses ranging from 0.75 – 1.75 g/kg. B. Typical plasma levels of ascorbate achieved in a single patient over time. Peak ascorbate levels post-infusion were approximately 500-fold higher than both baseline and pre-infusion trough ascorbate levels. C. Trough ascorbate levels after steady dosing regimen were significantly higher than baseline screening ascorbate levels. D. Ascorbate radical is observed in whole blood only with high levels of ascorbate. Ascorbate radical is below the limit of detection (<10 nM under these experimental conditions) in pre-infusion samples of whole blood that have typical nutritional levels of ascorbate, here 60 – 80 μM. Ascorbate radical (100 – 150 nM) is easily detectable in post-infusion samples that have very high levels of ascorbate (19 – 23 mM). This presence of ascorbate radical indicates the ongoing oxidation of ascorbate in whole blood (23).

Figure 3. Response to therapy

A. Baseline and post-treatment CT scans in a patient receiving ascorbate plus gemcitabine. The patient tolerated dose escalation of ascorbate, which stabilized at 75 g twice a week. Post-infusion analysis demonstrated plasma ascorbate concentrations of 22–27 mM (390 – 475 mg dL⁻¹) at 1 h. The patient had a 9-fold decrease in the primary tumor size within 4 months of treatment. B. Overall survival. Our phase I trial was designed to determine the effect of escalating doses of ascorbate when combined with gemcitabine in stage IV pancreatic cancer patients. The trial utilized a modified Burris regimen, administering gemcitabine for 3 weeks for each cycle of therapy along with ascorbate given twice weekly for every week. Historic median survival for gemcitabine-treated patients is 5.65 months (4). The mean survival is 12 months.

Figure 4. Pharmacological ascorbate does not increase markers of systemic oxidative damage

Baseline and post-treatment F₂-isoprostane levels in five patients receiving gemcitabine + ascorbate twice weekly to achieve peak plasma levels ≥ 20 mM (≥ 350 mg dL⁻¹). In all patients tested, the F₂-isoprostane level decreased after ascorbate infusions.

Figure 5. Pharmacological ascorbate does not alter the GSSG/2GSH redox buffer of RBCs

Blood was collected from patients before and immediately after infusion and assayed for the intracellular concentrations of GSH and GSSG in the RBCs; this information was used to assess the intracellular redox status as manifest by E_hc. Plotted in panels A through C are the intracellular concentrations for samples from three individuals and the corresponding calculated half-cell reduction potential (E_hc); the error bar represents measurement variability from triplicate determinations. In panel D, the relative average levels for all analytes are presented. Pre-infusion concentrations for each individual are normalized to 1, then the corresponding relative values for the post sample are calculated. The error bars represents differences between patients.