High-dose intravenous vitamin C, a promising multi-targeting agent in the treatment of cancer

Mounting evidence indicates that vitamin C has the potential to be a potent anti-cancer agent when administered intravenously and in high doses (high-dose IVC). Early phase clinical trials have confirmed safety and indicated efficacy of IVC in eradicating tumour cells of various cancer types. In recent years, the multi-targeting effects of vitamin C were unravelled, demonstrating a role as cancer-specific, pro-oxidative cytotoxic agent, anti-cancer epigenetic regulator and immune modulator, reversing epithelial-to-mesenchymal transition, inhibiting hypoxia and oncogenic kinase signalling and boosting immune response. Moreover, high-dose IVC is powerful as an adjuvant treatment for cancer, acting synergistically with many standard (chemo-) therapies, as well as a method for mitigating the toxic side-effects of chemotherapy. Despite the rationale and ample evidence, strong clinical data and phase III studies are lacking. Therefore, there is a need for more extensive awareness of the use of this highly promising, non-toxic cancer treatment in the clinical setting. In this review, we provide an elaborate overview of pre-clinical and clinical studies using high-dose IVC as anti-cancer agent, as well as a detailed evaluation of the main known molecular mechanisms involved. A special focus is put on global molecular profiling studies in this respect. In addition, an outlook on future implications of high-dose vitamin C in cancer treatment is presented and recommendations for further research are discussed.

Background

Vitamin C (VitC), also known as ascorbic acid or ascorbate, is an essential water-soluble vitamin that plays an important role in human physiology. Most of its physiological functions involve its ability to act as an antioxidant or as a cofactor for a wide variety of enzymatic reactions, thereby contributing to stabilisation of the tertiary structure of collagen, norepinephrine synthesis and iron absorption [1, 2]. Emerging data show that VitC is also a cofactor for newly characterised hydroxylases of the family of Fe-containing 2-oxoglutarate-dependent dioxygenases that regulate gene transcription and cell signalling pathways [3, 4]. In addition, immune cells accumulate high concentrations of VitC, underlining its key function in various processes within the immune system [5]. Importantly, while most vertebrate species can synthesize ascorbic acid, humans cannot, and they are therefore dependent on oral consumption of VitC.

The concept of utilizing VitC as a therapeutic agent for cancer care was first introduced by double Nobel Prize winning chemist Linus Pauling and physician Ewan Cameron almost 50 years ago [6,7,8]. Specifically, Pauling and Cameron published a number of clinical reports that indicated significantly prolonged survival rates of terminal cancer patients treated with pharmacological doses of VitC (10 g/ day by intravenous infusion for about 10 days and orally thereafter) compared to matched historical controls that did not receive VitC. The same amounts of high-dose VitC administered orally only in randomized double blind placebo control studies could not confirm this favourable response in advanced human cancer [9, 10]. Herein lies the essence of much controversy concerning the implementation of VitC in cancer treatment over the past decades. An important distinction must therefore be made between orally administered VitC (OC), achieving maximum plasma concentrations of no more than 220 μmol/L of blood, and pharmacological or high-dose IVC, generating plasma concentrations up into the millimolar range (≥ 15 mmol/L) [11,12,13], which is needed to kill cancer cells based on pre-clinical studies.

In light of this, high-dose IVC has re-emerged as a potent anti-cancer agent over the past two decades, with several phase I and a few phase II clinical trials reporting high tolerability and safety with promising signs of efficacy in the treatment of various cancer types, either as monotherapy or as a combination therapy [14,15,16]. In addition, there is strong clinical evidence for IVC’s ability to reduce chemotherapy-related side effects, such as fatigue, and to improve quality of life also in the palliative care setting [17,18,19].

The aim of this review is to create an up-to-date overview of the most important research conducted within the field of high-dose VitC and cancer therapy. First, the use of high-dose VitC mono- and combination therapy in the pre-clinical and clinical setting is discussed, followed by a discussion of the molecular mechanisms found to be involved in the anti-cancer activity portrayed by VitC. Specifically, the contribution of emerging global profiling studies based on proteomics, transcriptomics and metabolomics to these insights will be highlighted. In this regard, our findings will provide an outlook on future research, examining current gaps in our knowledge and addressing the limitations of research in the clinical setting and the need for more extensive clinical trials. Also, future implications of high-dose VitC in cancer therapy will be discussed in both treatment and palliative care.

High-dose VitC as a single agent

The pioneering clinical studies that initiated the interest in VitC as anticancer agent [6,7,8] employed VitC as single agent. Since then, a great number of clinical and pre-clinical studies have explored high-dose VitC. In this section, we briefly summarize the pre-clinical and clinical studies of VitC as monotherapy before elaborating more on the combination therapy studies.

Pre-clinical VitC monotherapy studies

A vast number of studies have shown encouraging anti-cancer activity of VitC at millimolar concentrations (~ 1–20 mM) in pre-clinical models of various cancer types [15]. The most investigated have been leukaemia [20,21,22,23,24], colon cancer [25,26,27,28,29,30,31,32], melanoma [33,34,35,36,37], pancreatic cancer [14, 31, 38] and prostate cancer [39,40,41]. Similar results have been described for the treatment of non-small-cell lung cancer (NSCLC) [16], breast cancer [31, 42], ovarian cancer [31, 43, 44], hepatocellular carcinoma [45, 46], malignant mesothelioma [47, 48], thyroid cancer [49, 50], oral squamous cell carcinoma [51], neuroblastoma [52] and glioma, including the difficult-to-treat glioblastoma multiform (GBM) [16, 53, 54].

One notable example of the progress in VitC pre-clinical research is the recent work in hard-to-treat Kirsten Rat Sarcoma Viral Oncogene Homolog (KRAS) driven tumours, such as KRAS mutant colorectal cancer (CRC) [25, 27, 32]. Based on prior studies by Yun et al. [32] and Aguilera et al. [25], Cenigaonandia-Campillo et al. [27] used elevated doses of VitC (5–10 mM) in KRAS mutant CRC tumours, both in vitro and in vivo. They showed that VitC was able to target common metabolic aberrancies by decreasing adenosine triphosphate (ATP) and glucose transporter 1 (GLUT-1) levels, as well as by dissipating the mitochondrial membrane potential, which could sensitize KRAS mutant CRC cells to current treatments such as chemotherapy. Given the importance of developing better treatments for patients with KRAS driven tumours, non-toxic combinations with VitC are also being explored and will be discussed in the following section 2.

In the majority of cancer types, most of the in vivo studies have shown inhibition of tumour growth (40–60%) by using elevated doses of ascorbate (1-4 g/kg) either intravenously (IV) or intraperitoneally (IP) [15, 55,56,57]. Importantly, in order to maintain VitC levels inside the tumour, daily administration is the most optimal schedule [56]. By using these doses and frequency, VitC also successfully reduced and/or impaired metastasis formation (50–90%) [33, 39, 43, 58,59,60,61].

In terms of safety and tolerability, several studies have shown that high-dose VitC does not increase toxicity levels in vivo yet protects from other treatment side-effects when used as an adjuvant agent [15, 62,63,64].

Overall, the studies performed in vitro and in vivo using high-dose VitC as single agent in a large number of cancer types, have shown that it is a promising anti-cancer agent impairing both tumor growth and metastasis.

Clinical VitC monotherapy studies

Clinical monotherapy studies administering high-dose VitC in patients with various types of advanced malignancies report this therapy to be safe, showing no significant toxicity at doses of up to 3 g/kg [13] (Table 2). These studies additionally demonstrated that at the given doses, ascorbate plasma levels of over 10 mM could be sustained for several hours, and observed maximum achievable blood concentrations of up to 49 mM [13]. Grade 3 or higher adverse events possibly related to IVC treatment were reported in only 1–2 cases per study (with 17–24 patients included per study, see Table 2), the most common being hypokalemia [13, 65], hypernatremia [13], hypertension and anemia [66]. Riordan et al. [65] additionally reported one case of kidney stones in a metastatic CRC patient with a history of renal calculi, suggesting IVC may be contraindicated for patients with renal dysfunction. Nielsen et al. [66] reported one case of pulmonary embolism and pneumonia each, both of which can also be attributed to the underlying disease, since cancer is known to increase the risk of thromboembolic events. Hoffer at al [12]. reported no grade 3 or higher toxicities.

Beyond being safe and well-tolerated, objective anti-tumor response was not observed in any of these IVC monotherapy studies. While Stephenson et al. [13], Hoffer et al. [12] and Riordan et al. [65] reported 3 (out of 16), 2 (out of 24) and 1 (out of 24), and patients with stable disease, respectively, the study by Nielsen et al. [66] reported no signs of disease remission or stabilization. Latter result is likely related to the fact that both dose and administration frequency (maximum of 60 g whole body dose given 1 time per week for 12 weeks) was considerably lower compared to the other studies (here, up to 3 g/kg were administered at least 3 times per week, for up to 8 weeks, see Table 2). That being said, a number of promising case reports have reported unexpectedly long survival time and in some cases even complete tumour regression of advanced or metastatic disease [67,68,69,70,71,72]. In future studies, molecular profiling of these exceptional responders would be of high value to explore molecular features that make certain tumors more sensitive to IVC.

Currently, one phase II study is ongoing whereby the effect of high-dose (1.25 g/kg) VitC monotherapy is being studied in resectable or metastatic colorectal, pancreatic and lung tumors (Table 3). The objective of the study is to investigate the effect on pathological tumor response in resectable tumors and to observe objective tumor response in KRAS or BRAF mutant metastatic tumors (NCT03146962) [73]. In addition, one medium-dose effort in bladder cancer (NCT04046094) [74] as well as several oral and/or low-dose monotherapy studies in non-solid tumors (NCT03682029)(NCT03613727)(NCT03964688) [75,76,77] are currently ongoing in line with the promising pre-clinical data concerning these latter cancer types [21, 78].

In general, high-dose VitC monotherapy has not been clinically assessed in patients that have not received (heavy) prior systemic treatment and that are not terminally ill. This fact may explain the limited response effects observed. Finding a feasible clinical setting to include less heavily pre-treated patients however is complicated, as it would involve denying patients standard of care. For this reason, future applications of high-dose VitC as cancer therapy may rather be in combination strategies and we will focus more on this application in the sections below. However, important lessons regarding administration frequency can be learned from these monotherapy studies, whereby only those studies that administered IVC at least 3 times per week warranted further clinical trials. The recommended doses ranged from 1.5 g/kg [12] to 1.9–2.2 g/kg [13].

VitC monotherapy in palliative care and quality of life

In palliative care, high-dose VitC is currently gaining ground due to its highly safe and tolerable profile. Not only is high-dose VitC known to relieve pain in cancer patients [79], vast clinical evidence suggests that it has a significant positive impact on patients’ well-being [14, 17,18,19, 63, 80,81,82,83]. This might be due to the frequent hypovitaminosis and VitC deficiency in cancer patients [79, 84, 85], which are commonly enhanced by anti-neoplastic treatments [18].

For instance, a retrospective, multicentre, epidemiological cohort study [18] showed amelioration of appetite, fatigue, depression and sleep disorders in breast cancer and terminal cancer patients suffering from a wide variety of cancer types that received complementary 7.5 g IVC while being treated by respective standard regimens. More recently, a single-center, parallel-group, single-blind interventional study also in breast cancer patients [86] showed a similar and significant reduction of symptoms such as nausea, fatigue, tumor pain and loss of appetite by administering 25 g of IVC per week in addition to their current standard treatment. Favourably, no new side effects were reported after initiation of IVC treatment.

Moreover, another retrospective study showed that patients with radiotherapy-resistant bone metastasis did not only have less pain and better performance measures when given high-dose VitC, they had a median survival time of 10 months as compared to the 2 months median survival time within the control group [80].

Overall, high dose VitC administered as a single agent has not only been shown to be safe and well-tolerated in cancer patients, but also to ameliorate pain and to improve quality of life in the palliative care setting.

High-dose VitC in combination treatments

Many studies in the past years have investigated high dose VitC as an adjuvant pro-oxidative agent mainly in chemo- and radiotherapy. In addition, other combination treatments have been investigated as well. In this section, we review the pre-clinical and clinical literature of high dose VitC in combination treatments.

For pre-clinical studies, we provide detailed information per study and per combination (i.e. cancer type, VitC doses, route of administration, sample size, etc), and describe the observed effects such as synergism, enhanced efficacy and/or reduced toxicity (Table 1, Figs. 1, 2). Particularly for clinical studies, completed and on-going trials using IVC as monotherapy and combination treatment are described in detail (Tables 2, 3, Fig. 3). We examine relevant information on phase of study, type of interventions, IVC dose, injection scheme and number of patients enrolled. In addition, results of completed studies and primary outcomes of ongoing trials are thoroughly discussed.