BackgroundActivated platelets exert a pro-inflammatory action that can be largely ascribed to their ability to interact with leukocytes and modulate their activity. We hypothesized that platelet activation and consequent formation of monocyte-platelet aggregates (MPA) induces a pro-inflammatory phenotype in circulating monocytes.Methodology/Principal FindingsCD62P+ platelets and MPA were measured, and monocytes characterized, by whole blood flow cytometry in healthy subjects, before and two days after receiving influenza immunization. Three monocytic subsets were identified: CD14+CD16−, CD14highCD16+and CD14lowCD16+. The increase in high sensitivity C-reactive protein post-immunization was accompanied by increased platelet activation and MPA formation (25.02±12.57 vs 41.48±16.81; p = 0.01), along with enhancement of circulating CD14highCD16+ cells (4.7±3.6 vs 10.4±4.8; p = 0.003), their percentage being linearly related to levels of CD62P+-platelets (r2 = 0.4347; p = 0.0008). In separate in vitro experiments, co-incubation of CD14+CD16− cells, isolated from healthy donor subjects, with autologous platelets gave rise to up-regulation of CD16 on monocytes as compared with those maintained in medium alone (% change in CD14+CD16+ cells following 48 h co-incubation of monocytes with platelets was +106±51% vs monocytes in medium alone; p<0.001). This effect correlated directly with degree of MPA formation (r2 = 0.7731; p<0.0001) and was associated with increased monocyte adhesion to endothelial cells. P-selectin glycoprotein ligand-1 (PSGL-1) blocking antibody, which abrogates MPA formation, abolished these effects, as did the cyclooxygenase (COX)-2 selective inhibitor NS-398, aspirin and the EP1/EP2-selective antagonist AH6809.Conclusions/SignificanceThese data suggest that MPA formation, as occurs in the blood under pro-inflammatory conditions, expands the pool of circulating CD14highCD16+ monocytes in a COX-2 dependent manner, and these monocytes exhibit increased adhesion to endothelium. Our findings delineate a novel mechanism underlying the pro-inflammatory effect of platelet activation.

In oncology, preclinical and early clinical data increasingly support the use of a number of candidate “non-cancer” drugs in an off-label setting against multiple tumor types. In particular, metabolically targeted drugs show promise as adjuvant chemo and radiosensitizers, improving or restoring sensitivity to standard therapies. The time has come for large scale clinical studies of off-label drugs in this context. However, it is well recognized that high-cost randomized controlled trials may not be an economically viable option for studying patent-expired off-label drugs. In some cases, randomized trials could also be considered as ethically controversial. This perspective article presents a novel approach to generating additional clinical data of sufficient quality to support changes in clinical practice and relabeling of such drugs for use in oncology. Here, we suggest that a pluralistic evidence base and triangulation of evidence can support clinical trial data for off-label drug use in oncology. An example of an off-label drug protocol brought to the clinic for glioblastoma patients is presented, along with preliminary retrospective data from the METRICS study (NCT02201381). METRICS is a novel participant-funded, open-label, non-randomized, single-arm real-world study designed to gather high-quality evidence on the safety, tolerability, and effectiveness of four off-label metabolically targeted medicines as an adjunctive cancer treatment for glioblastoma patients.

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