1. Introduction

This paper aims to extend application of the movement repurposing already-marketed non-oncology drugs as adjuncts to current cancer treatments. The presented regimen, TICO, (tadalafil, isotretinoin, colchicine, omega-3 fatty acids) uses four FDA approved non-oncology drugs to partially reverse the characteristic immunosuppressive elements common in glioblastoma and in cancers generally.

First the paper will review the collected evidence that (A) the circulating, blood neutrophil-to-lymphocyte ratio (NLR) is elevated in all the common metastatic cancers, (B) that a higher NLR is associated with shorter survival across the common cancers, (C) that the NLR itself is retarding of immune responses, (D) that elevated NLR is associated with elevated numbers and function of myeloid-derived suppressor cells (MDSC), and that (E) normal neutrophils commonly also contribute to malignant growth in most cancers.

Secondly, this paper presents data on four already marketed generic drugs from general medical practice showing that they individually can lower the NLR in humans.

Neutrophils are viewed today as a “double edged sword” in that we have data on an anti-malignant cell activity of neutrophils, as well as a (much larger) database on a growth facilitating role of neutrophils contributing to malignant growth [1,2,3]. The latter is the subject of this paper.

Three nuances of note here: (A) A given cell, signaling system, or physiological element need not be either cancer growth facilitating or growth inhibiting. More commonly in biological systems both effects or attributes are simultaneously present. We speak of growth inhibiting or facilitating when one clearly predominates. (B) Research does not yet reveal determinants of neutrophils’ role in malignancy–growth facilitation or inhibition or neither. (C) The uniformity of higher NLR being associated with shorter survival across the common cancers, as in Table 1, tends to indicate that growth facilitation predominates. That neutrophils contribute to malignant growth in most cancers seems clear but how exactly they do so is not [4,5,6,7,8,9]. This paper will outline some of the putative pathways.

This paper is a follow up on, and an updating of Draghiciu et al., 2015’s paper and Wang et al., 2020’s paper where they presented data on a range of already-marketed drugs that had an ancillary attribute of lowering NLR [10,11]. TICO includes two of the 21 drugs reviewed by Wang et al. and Draghiciu et al.–a retinoid and tadalafil.

The regimen presented here, TICO, consists of the phosphodiesterase 5 (PDE5) inhibitor tadalafil, used to treat inadequate erections; isotretinoin, the retinoid used for acne treatment; colchicine, a standard gout (podagra) treatment; and the common fish oil supplement omega-3 polyunsaturated fatty acids, hereafter abbreviated simply as omega-3. These, individually or taken all together, would be expected to impose a low side effect burden, pose a very low risk for serious adverse reactions, and be relatively cheap. They each individually have demonstrated the ability to lower the NLR in humans, as in the representative data reviewed below. TICO therefore has potential to convert a high risk high NLR cancer to a lower risk category by virtue of TICO lowering of the NLR.

No other current standard laboratory blood test is as consistently abnormal across the common cancers as is the association between an elevated peripheral blood NLR and shorter overall survival. Table 1 lists representative reviews of this association in 18 common cancers. There are many other similar reviews and meta-studies showing the same shorter cancer survival associated with higher NLR. All these reviews covered hundreds of previous individual clinical studies on this association over the last decade. Several dozen reviews and meta-studies over the last decade have amply documented the general association of higher NLR with shorter survival in a wide range of the common non-hematological cancers [12,13,14,15,16,17,18].

During the differentiation process, usually in bone marrow, neutrophils’ nuclei evolve from round to banded and then to the common (normal) lobulated morphology [36]. Secretory granules, found mostly in mature lobulated neutrophils, store elastase, myeloperoxidase, cathelicidins, defensins, VEGF, and matrix metalloproteinases, i.e., granulocyte-colony stimulating factor, G-CSF, or granulocyte macrophage-colony stimulating factor, GM-CSF, together referred to as G(M)-CSF, initiate a release process of neutrophils from marrow.

Only 1 or 2% of the body’s neutrophils reside in circulating blood. A total of 90% reside in bone marrow. The cytokine CXCL12 is expressed in large amounts by marrow stroma. This retains developing neutrophil lineage cells in bone marrow via neutrophils’ surface expression of CXCL12′s receptor CXCR4. G(M)-CSF reduces stromal expression of CXCL12, thereby tending to release the previously held neutrophils [37,38]. See Figure 1 for a simplified diagram of this process.

G(M)-CSF exposure also reduces neutrophil lineage cells’ expression of CXCR4 further contributing to release from marrow. Multiple other cytokines and hormones participate in neutrophils’ exodus or retention from marrow. Multiple counteracting cytokines and hormones act to retain neutrophils in marrow. What happens—release or retention—is the result of the balance between these many forces.

Figure 1 schematically depicts tumor synthesized G(M)-CSF mediating reduction of neutrophil lineage cells’ retention in bone marrow, resulting in an increase in circulating and tumor resident MDSC as well as an absolute neutrophilia. Arg-1 (and other soluble mediators) synthesized by MDSC is locally suppressive of T cells, these processes accounting at least partially for the higher NLR seen across the common cancers.

There exists a suite of neutrophil related elements that each have a literature database showing their contributions to malignant growth, acting both individually and as an ensemble. This neutrophil centered, interacting, multicomponent inflammation system consists of cancer cell synthesized G(M)-CSF [39,40,41], neutrophil extracellular traps (NETs) [42,43], activation of the NLRP3 inflammasome with consequent increases in MDSC functions [44,45,46], IL-1beta activation [47,48], normal mature neutrophils, and the higher NLR. As an ensemble, these elements function together to fight infection and contribute to wound healing but in cancer become pathologically engaged, facilitating tumor growth and metastasis [49,50]. Core elements of this neutrophil centered system are diagrammed in Figure 2. Table 2 gives quick reference definitions of these elements.

The reviewed data in this paper lead to three conclusions: (1) that MDSC, NLR, and absolute neutrophil count tend to be associated with enhanced tumor growth across the common cancers, and (2) that several well-tolerated, already marketed drugs from general medical practice, the TICO drugs, have evidence that they can lower the NLR and (3) the risk–benefit ratio favors a clinical study of TICO as adjunct to current standard treatments, particularly those using the immune checkpoint inhibitors.

2. The NLR

Humans’ normal NLR tends to be about 2, but values between 1 and 3 can be seen in healthy people [51,52,53]. Age and gender have only minor effects on the NLR. NK lymphocyte count tends to vary inversely with the NLR [52].

Absolute numbers of circulating neutrophils are commonly elevated in cancers generally, where higher numbers are associated with shorter survival [13,54,55,56,57]. Several recent reviews together make a strong empirical case for important growth promotion by neutrophils in the common cancers [13,57,58,59,60,61,62,63,64,65]. However, in addition to the relative numbers to lymphocytes, the NLR seems to be determinative even in presence of non-elevated absolute neutrophil numbers. This may reflect an anti-lymphocyte function of a neutrophil subset. Section 4 below outlines how the neutrophil MDSC subset facilitates tumor growth by damaging local T cells.

High NLR also occurs in the common inflammatory conditions—gout, [66]; Crohn’s disease [67]; COVID-19, [68]; SLE, [69]; rheumatoid arthritis, [70]; erosive osteoarthritis [71]; ankylosing spondylitis, [72]; psoriasis and psoriatic arthritis, [73]; ischemia [74]; and others.

Although we have hundreds of primary studies in human cancers showing abnormally elevated NLR, of which Table 1 is representative, we cannot conclude from this that cancer is an inflammatory disease. However, we might be able to conclude that there is some kind of inflammatory process comprising or contributing to the pathophysiology that facilitates malignant growth.

The association of higher NLR with shorter survival holds particularly prominently in cancers treated with immune checkpoint inhibitors, pointing to an association of higher NLR with greater immunosuppression [75,76,77,78,79,80,81,82,83]. As typical examples, Stares et al. reported pembrolizumab (Keytruda™), a pharmaceutical monoclonal antibody to PD-1, used in non-small cell lung cancer gave an overall survival of 8 months if NLR was >5 but 21 months if NLR was <5, and Muhammed et al.’s study in hepatocellular carcinoma treated with various similar checkpoint inhibitors that showed an overall survival of 8 months if NLR was >5 and 18 months if NLR was >5 [81,84]. Multiple other studies of the NLR effect on immune checkpoint inhibitors showed similar results.

Further primary studies on the association of high NLR with lower survival time in these individual cancers have come out since the listed reviews below were published. The listed reviews below summarize hundreds of primary studies documenting that higher NLR correlates with a shorter than average survival in their reviewed cancer. The importance of how uniform this relationship is across multiple cancers is underappreciated in the oncology community. Many concordant references to meta-studies could be listed for each entry but only a single recent review of past work is listed.

3. G(M)-CSF

Both G-CSF and GM-CSF are 15 to 20 kDa glycoproteins central to, but not essential for, bone marrow hematopoiesis. G(M)-CSF acts (1) as a direct growth factor acting on cell surface receptors on malignant cells or, (2) as an indirect growth factor by increasing MDSC, or (3) by increasing numbers of normal mature neutrophils that provide trophic and angiogenic factors [3,85,86]. Tumor synthesized G(M)-CSF has been recognized for over a decade now as an element driving immunosuppression in cancer [87].

Common metastasizing cancers tend to be whole body diseases. Abnormal and pathophysiologically relevant bone marrow or spleen hematopoiesis is driven in cancer in part by tumor-synthesized G(M)-CSF [40,41,87,88,89,90,91]. The net tendency of excess G(M)-CSF, whether malignant tissue produced or exogenous as a pharmacological agent, is to hasten transit time through the stages of granulopoiesis, resulting in increased neutrophil release, but also release of less mature neutrophils which are thought to comprise the MDSC subset. Retention in marrow is the result of many factors or retention axises, several of which are disrupted by G(M)-CSF [1,92]. Two of these retention pathways are diagrammed in Figure 1.

However, other cytokines, such as IL-1beta, IL-8, and IL-6, are also involved, and granulopoiesis will proceed even in absence of G-CSF. There may also be immune response reducing effects directly on T cells via a G-CSF receptor pathway on T cells [92].

Multiple stimuli trigger increased G(M)-CSF synthesis by endothelial cells, fibroblasts, macrophages, monocytes, and bone marrow stroma. These stimuli—examples are vascular endothelial growth factor (VEGF), IL-1beta, IL-17, bacterial lipopolysaccharides, TNF-alpha, i.e.,—act via different pathways to result in increased G(M)-CSF. The stimuli for malignant tumor synthesis and secretion have not been established but the correlation between a tumor’s higher production of G(M)-CSF and more aggressive clinical course is well established [40,41,87,88,89,90,91,92,93].

The partially separate matter of monocytic lineage cells’ MDSC will not be discussed here. Malignancy associated increases in circulating MDSC tend to be of granulocyte-MDSC subtype [87,88,89,90,91,92,93].

In several transplantation models G-CSF promoted expansion of MDSCs and enhanced their suppressive function against T cell proliferation [94,95]. Breast cancer cell secreted GM-CSF drove immunosuppression in vitro by increasing MDSC Arg-1 expression thereby inhibiting T cell function, human breast cancer biopsy tissue reflecting this as well [96]. Placental origin G-CSF upregulates systemic MDSC numbers and pharmaceutical G-CSF increases MDSC during transplantation related neutrophil mobilization [97]. Collected data on G-CSF mediated increases in circulating MDSC was recently reviewed [91].

4. MDSC

Human MDSC have been variously defined but are commonly thought of as monocytic, M-MDSC CD11b + CD14 + CD15 − CD33 + HLA−DR−, or granulocytic, granulocyte-MDSCs, CD11b + CD14 + CD15 + (or CD66b+) CD33+LOX-1+ [98,99,100,101]. In cancer, 79–80% of MDSC are of the granulocyte-MDSC subset, the remainder are M-MDSC. Elevation of circulating MDSC is a uniform finding in human cancers [102,103,104,105].

Multiple signaling pathways drive MDSC expansion. Examples are signaling chains using VEGF, G(M)-CSF, IL-6, IL-1beta, TLR, high lactate levels, and TNF. Note also the potential for positive feedback loops within the chain of mediators leading to MDSC increases as depicted in Figure 2.

There is an intrinsic, direct relationship between an elevated NLR and increased numbers and function of both circulating and tumor-resident MDSC [109,110,111,112,113,114,115,116].

In clinical biopsies, tumor Arg-1 expression is localized to granulocytic myeloid cells [117]. Low numbers of granulocytic-MDSC uniformly predict a longer overall survival during treatment with Is ipilimumab, nivolumab, or pembrolizumab in melanoma [118], in epithelial ovarian cancer [119], and in non-small cell lung cancer [120,121].

Concordant with the notion that NLR and MDSC are positively correlated, lower NLR predicts a better response to, and longer survival during, treatment with immune checkpoint inhibitors in melanoma [122], in non-small cell lung cancer [123,124], in clear cell renal carcinoma [125], head and neck squamous cell carcinoma [126,127], urothelial carcinoma [128,129], cervical cancer [130], and hepatocellular carcinoma [131].

It is the contention of this paper on TICO that, based on data reviewed above, the activated NLRP3 inflammasome, MDSC, G(M)-CSF form a constellation, a triad that is at least partially responsible for the elevated NLR and NETs that we see in cancer. As diagrammed in Figure 1, tumor synthesized G(M)-CSF results in release of greater numbers of MDSC that are both trophic for cancer cells but also T cell suppressing via over synthesis of Arg-1 and other soluble mediators, thereby skewing the NLR to higher ratios. The activated NLRP3 inflammasome is one of the signaling hubs connecting the constellation [46,132,133]. The TICO drugs aim to disrupt this cancer growth enhancing constellation.

5. The TICO Drugs

As is the case with the other multi-drug regimens, individual dosing based on tolerance will be required. Clinical study of stepwise addition of the TICO drugs, titrated to tolerance and reduction of NLR in metastatic cancer alongside standard current treatment will tell us if pharmacological lowering of NLR will improve survival or not.

As with all the individual drugs in similar multi-drug regimens, as we outlined in past work, we would not expect any single drug or intervention added to standard treatment to strongly lengthen survival [134,135,136]. As we have outlined previously, absent a “silver bullet” and identification of a single element that drives all the multiple attributes of malignancy, a multidrug approach will be needed to prolong life in the common metastatic cancers [63]. TICO attempts to address one of the several pathological engagements of normally functioning body systems that are pathologically engaged to facilitate growth–in TICO, the bone marrow, and neutrophil related contributions.

The mechanism of action in lowering NLR of TICO regimen drugs have not always been fully delineated. The focus below on TICO drugs emphasizes evidence of their empirical effect on NLR in clinical use more than their mechanism of achieving that reduction.