Ascorbic acid as a pharmacologic agent in disease treatment

解释 口服时，抗坏血酸受到三种生理机制的严格控制：吸收、组织转运和肾脏重吸收/排泄。静脉注射抗坏血酸绕过了严格的控制，直到肾脏恢复动态平衡，正如本实验室进行的生物利用度实验所确定的那样。这些数据对癌症治疗具有令人惊讶和新颖的影响。 30多年前，伊万·卡梅伦提出抗坏血酸可能对治疗癌症患者有有益作用。与两届诺贝尔奖得主莱纳斯·鲍林一起，他们发表了两个病例系列，表明每天大剂量10克抗坏血酸对一些晚期癌症患者有潜在好处。为便于比较，请注意，当时抗坏血酸的推荐膳食摄入量为每天60毫克，或治疗剂量的0.6%。卡梅隆-鲍林的数据受到了批评，因为它们是回溯性的，没有安慰剂对照，部分是主观的，缺乏独立的病理证实。梅奥诊所的研究人员进行了两项双盲、安慰剂对照试验，每天使用10克抗坏血酸治疗晚期癌症患者，但没有发现任何效果。基于这些数据，医生被强烈建议不要在癌症治疗中使用抗坏血酸。根据我们广泛的生物利用度数据，我们回顾了卡梅伦和他的同事以及梅奥诊所的研究人员的实验。我们惊讶地发现，与卡梅伦相关的患者接受了静脉注射和口服抗坏血酸，而与梅奥诊所相关的患者只接受了口服抗坏血酸。因此，治疗组之间的比较是无效的，抗坏血酸在癌症治疗中的问题需要重新评估。 根据我们的生物利用度数据和药代动力学模型，静脉注射抗坏血酸可以产生比最大耐受量高70倍的血浆浓度。在体外，很容易在人类身上达到的药理抗坏血酸浓度可以杀死癌症，但不能杀死正常细胞。杀伤是由细胞外抗坏血酸、其氧化为抗坏血酸自由基以及依赖蛋白质的过氧化氢的形成所介导的。为了在没有抗坏血酸干扰的情况下检测过氧化氢，实验需要专门的化学合成过氧杂多酮，这在商业上是买不到的。基于获得的数据，我们提出并在体内验证了药理抗坏血酸是优先在血管外空间形成抗坏血酸自由基的药物，而不是血液中的抗坏血酸自由基。再一次，活体实验依赖于过氧杂多酮的合成。 同时，我们提出了一个全球假说，解释了为什么癌细胞而不是正常细胞对抗坏血酸通过过氧化氢形成介导的死亡敏感。在抗坏血酸和催化金属存在下，过氧化氢会导致多种活性氧物种(ROS)的形成。由此形成的活性氧物种可以通过多种机制之一选择性地杀死癌细胞：激活多聚ADP-核糖聚合酶(PARP)和/或耗尽细胞内的ATP；产生不能足够快地修复以防止细胞死亡的DNA损伤；通过谷胱甘肽依赖的过氧化物酶消耗细胞内的还原等价物而耗尽；直接线粒体毒性；直接膜毒性。探索这些概念的实验正在进行中。 目前的其他努力集中在：在动物模型中继续研究药理抗坏血酸作为抑制肿瘤生长的试剂；过氧化氢的体内成像；启动和继续第二阶段临床试验，以及临床使用模式和安全性的持续表征。I期临床试验很有希望，到目前为止的安全性数据也是如此。

Explanation When ingested orally, ascorbic acid is tightly controlled by three physiologic mechanisms: absorption, tissue transport, and renal reabsorption/excretion. Intravenous administration of ascorbic acid bypasses tight control until the kidney restores homeostasis, as determined from bioavailability experiments conducted by this laboratory. These data have surprising and novel implications for cancer treatment. More than 30 years ago, Ewan Cameron proposed that ascorbic acid might have a beneficial effect in treating patients with cancer. Joined by the two-time Nobel Laureate Linus Pauling, they published two case series indicating potential benefit of a large daily dose of ascorbic acid, 10 grams, in some patients with terminal cancer. Note for comparison purposes that the recommended dietary allowance for ascorbic acid at that time was 60 mg daily, or 0.6% of the treating dose. The Cameron-Pauling data were criticized because they were retrospective, without placebo control, in part subjective, and lacked independent pathologic confirmation. Investigators at the Mayo Clinic conducted two double blind, placebo controlled trials using 10 grams ascorbate daily to treat patients with advanced cancer, and found no effect. Based on these data, physicians were strongly advised to not use ascorbic acid in cancer treatment. In light of our extensive bioavailability data, we reviewed the experiments of Cameron and colleagues, and the Mayo Clinic investigators. We were astonished to recognize that the Cameron-associated patients received IV and oral ascorbic acid, but the Mayo Clinic-associated patients received only oral ascorbic acid. Thus, comparisons between the treatment groups were invalid, and the issue of ascorbic acid in cancer treatment needed re-evaluation. Based on our bioavailability data and pharmacokinetics modeling, intravenous ascorbic acid dosing can produce plasma concentrations as much as 70 fold higher than maximally tolerated oral doses. In vitro, pharmacologic ascorbic acid concentrations that are easily achieved in humans kill cancer but normal cells. Killing is mediated by extracellular ascorbic acid, its oxidation to ascorbate radical, and protein-dependent formation of hydrogen peroxide. To detect hydrogen peroxide without ascorbate interference, experiments required specialized chemical synthesis of peroxyxanthones, not commercially available. Based on the obtained data, we proposed and validated in vivo the hypothesis that pharmacologic ascorbic acid is a pro-drug for preferential formation of ascorbate radical in the extravascular space, but not blood. Again, in vivo experiments were dependent on synthesis of peroxyxanthones. Concurrently, we developed a global hypothesis explaining why cancer but not normal cells are sensitive to ascorbate mediated death via hydrogen peroxide formation. Hydrogen peroxide in the presence of ascorbate and catalytic metals leads to formation of multiple reactive oxygen species (ROS). Reactive oxygen species thus formed may selectively kill cancer cells by one of many mechanisms: activation of poly ADP-ribose polymerase (PARP) and/or depletion of intracellular ATP; generation of DNA damage that cannot be repaired quickly enough to prevent cell death; depletion of intracellular reducing equivalents via their consumption by glutathione-dependent peroxidases; direct mitochondrial toxicity; direct membrane toxicity. Experiments exploring these concepts are underway. Other current efforts are focused on: continued investigation of pharmacologic ascorbic acid as an agent to inhibit tumor growth in animal models; in vivo imaging of hydrogen peroxide; initiation and continuation of phase II clinical trials, and continued characterization of clinical usage patterns and safety. Phase I clinical trials are promising, as are safety data to date.