Ascorbic acid as a pharmacologic agent in disease treatment

药理抗坏血酸 解释 口服时，抗坏血酸受到四种生理机制的严格控制：吸收、组织转运、肾脏重吸收/排泄和利用。静脉注射抗坏血酸绕过严密的控制，直到肾脏恢复动态平衡，正如本实验室进行的药代动力学和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。 大约50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的作用。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，表明每天大剂量的抗坏血酸10克(10,000毫克)对一些晚期癌症患者有潜在的好处。为了进行比较，请注意，当时推荐的抗坏血酸饮食摄入量为每天60毫克，或治疗剂量的0.6%。卡梅伦-鲍林的报告受到了批评，因为它们具有追溯性，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。根据我们广泛的药代动力学和生物利用度数据，我们回顾了卡梅伦和他的同事以及梅奥诊所研究员的实验。我们是第一个认识到卡梅隆相关患者接受静脉注射和口服抗坏血酸，但梅奥诊所相关患者只接受口服抗坏血酸。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、药代动力学模型和在接受静脉注射剂量的人中的测量，静脉注射抗坏血酸可以产生药理血浆浓度。这些浓度比最大耐受量高出1000倍。在体外，很容易在人体内达到的药理抗坏血酸浓度可以杀死癌细胞，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。基于所获得的数据，我们提出并在体内验证了药理抗坏血酸是优先在血管外间隙形成过氧化氢的前体药物，而不是在血液中。 从这些发现，我们开发并继续测试一个全球假说，解释为什么癌细胞，而不是正常细胞，对通过细胞外过氧化氢形成的抗坏血酸介导的死亡敏感。基本的先决条件是在药理抗坏血酸存在的情况下形成细胞外过氧化氢。然后，细胞外的过氧化氢可能通过扩散进入细胞。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；与细胞内铁库的相互作用。其他的机制被预测并且可能，都是基于细胞外过氧化氢产生细胞外和细胞内的活性氧物种。探索这些概念的实验仍在继续。 许多其他实验室的发现证实了我们最初的观察结果。此外，一些发现表明，抗坏血酸可能对某些癌症具有作用机制，这些机制可能与细胞外过氧化氢的形成没有直接关系。例如，抗坏血酸作为辅助因子增加了10-11易位酶的活性，导致DNA去甲基化和羟甲基化增加。抗坏血酸如何在生理或药物浓度下调节这些作用，以及细胞外或细胞内过氧化氢的产生是否改变这些作用，对抗坏血酸的使用具有临床意义。 目前的其他努力集中在：继续研究药理抗坏血酸作为抑制肿瘤生长的药物在选定治疗方法的动物模型中的作用；在人体内测量过氧化氢；启动和继续进行II期临床试验；药理抗坏血酸在其他疾病，如败血症和新冠肺炎中的应用；以及继续表征临床使用模式和安全性。一期和二期临床试验前景看好，迄今为止的安全性数据也是如此。

Pharmacologic Ascorbate Explanation When ingested orally, ascorbic acid is tightly controlled by four physiologic mechanisms: absorption, tissue transport, renal reabsorption/excretion, and utilization. Intravenous administration of ascorbic acid bypasses tight control until the kidney restores homeostasis, as determined from pharmacokinetics and bioavailability experiments conducted by this laboratory. These data have surprising and novel implications for cancer treatment. Approximately 50 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 that suggested potential benefit of a large daily dose of ascorbic acid, 10 grams (10,000 mg), in some patients with terminal cancer. For comparison purposes, note 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 reports 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 pharmacokinetics and bioavailability data, we reviewed the experiments of Cameron and colleagues, and the Mayo Clinic investigators. We were the first 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, pharmacokinetics modeling, and measurements in people who received intravenous doses, intravenous ascorbic acid can produce pharmacologic plasma concentrations. These concentrations are up to 1000-fold higher than maximally tolerated oral doses. In vitro, pharmacologic ascorbic acid concentrations that are easily achieved in humans kill cancer cells but not 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 hydrogen peroxide in the extravascular space, but not in blood. From these findings, we developed and continue to test a global hypothesis explaining why cancer cells, but not normal cells, are sensitive to ascorbate mediated death via extracellular hydrogen peroxide formation. The essential prerequisite is formation of extracellular hydrogen peroxide in the presence of pharmacologic ascorbic acid. Extracellular hydrogen peroxide is then able to enter cells, likely by diffusion. 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; interaction with intracellular iron stores. Additional mechanisms are predicted and likely, all based on generation of extracellular and intracellular reactive oxygen species from extracellular hydrogen peroxide. Experiments exploring these concepts continue. Findings from many other laboratories have confirmed our original observations. Additionally, some findings suggest that ascorbate may have mechanisms of action on some cancers that may not be directly related to extracellular hydrogen peroxide formation. For example, ascorbate as a co-factor increases activity of the ten-eleven translocation enzymes, causing DNA demethylation and an increase in hydroxy-methylation. How ascorbate mediates these actions, whether at physiologic or pharmacologic concentrations, and whether extracellular or intracellular hydrogen peroxide generation modifies such actions, has clinical implications for ascorbate use. Other current efforts are focused on: continued investigation of pharmacologic ascorbic acid as an agent to inhibit tumor growth in animal models with select treatment modalities; in vivo measurement of hydrogen peroxide in humans; initiation and continuation of phase II clinical trials; use of pharmacologic ascorbic acid in other diseases such as sepsis and COVID-19; and continued characterization of clinical usage patterns and safety. Phase I and Phase II clinical trials are promising, as are safety data to date.