Cure Alveolar Soft Part Sarcoma International (iCureASPS)

Radiosynthesis and validation of (±)-[18F]-3-fluoro-2-hydroxypropionate ([18F]-FLac) as a PET tracer of lactate to monitor MCT1-dependent lactate uptake in tumors


Abstract

Cancers develop metabolic strategies to cope with their microenvironment often characterized by hypoxia, limited nutrient bioavailability and exposure to anticancer treatments. Among these strategies, the metabolic symbiosis based on the exchange of lactate between hypoxic/glycolytic cancer cells that convert glucose to lactate and oxidative cancer cells that preferentially use lactate as an oxidative fuel optimizes the bioavailability of glucose to hypoxic cancer cells. This metabolic cooperation has been described in various human cancers and can provide resistance to anti-angiogenic therapies. It depends on the expression and activity of monocarboxylate transporters (MCTs) at the cell membrane. MCT4 is the main facilitator of lactate export by glycolytic cancer cells, and MCT1 is adapted for lactate uptake by oxidative cancer cells. While MCT1 inhibitor AZD3965 is currently tested in phase I clinical trials and other inhibitors of lactate metabolism have been developed for anticancer therapy, predicting and monitoring a response to the inhibition of lactate uptake is still an unmet clinical need. Here, we report the synthesis, evaluation and in vivo validation of (±)-[18F]-3-fluoro-2-hydroxypropionate ([18F]-FLac) as a tracer of lactate for positron emission tomography. [18F]-FLac offers the possibility to monitor MCT1-dependent lactate uptake and inhibition in tumors in vivo.



https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5421858/

INTRODUCTION

Metabolic plasticity is a hallmark of cancer cells allowing them to optimally use existing resources for energy production and biosynthesis. Among possible fuels, lactate singles out as it is at the core of a metabolic cooperation between glycolytic cancer cells that produce lactate and oxidative cancer cells that use it [1]. This cooperation is of symbiotic nature: by delivering lactate to oxidative cancer cells that have a metabolic preference for lactate compared to glucose, glycolytic cancer cells facilitate glucose diffusion and use in the hypoxic/glycolytic cancer compartment [1–4]. Oxidative cancer cells, in turn, use lactate oxidation to promote autophagy, which has been linked to resistance to oxidative stress [5]. Together with other processes such as commensalism, autophagy and cannibalism, metabolic cooperativity represents an evolutionary solution for cell survival and proliferation in a metabolically altered environment [6].

At physiological pH, lactic acid (pKa 3.86) is fully dissociated in lactate and proton. Consequently, lactate swapping between glycolytic and oxidative cancer cells primarily depends on the expression and activity of lactate transporters of the monocarboxylate transporter (MCT) family that are located at the cell membrane [4, 7]. MCTs are passive transporters, among which MCT1 to MCT4 can transport lactate and are driven by the transmembrane gradient of lactate and protons. MCT4/SLC16A3, which has the lowest affinity for lactate (Km 22-28 mM) but a high turnover rate, is well adapted to facilitate the export of lactate and protons by glycolytic cancer cells [8–10]. This isoform is hypoxia-inducible (SLC16A3 is a direct target gene of hypoxia-inducible factor-1 [HIF-1]) [11] and does not efficiently transport pyruvate (Km 153 mM) [4, 8, 12]. Comparatively, MCT1/SLC16A1 has a higher affinity for lactate (Km 3.5-10 mM) and can efficiently transport pyruvate (Km 1 mM) and ketone bodies [4, 12]. Although SLC16A1 is not a direct HIF-1-target gene [11], experimental evidence showed that MCT1 expression can be induced by hypoxia in a HIF-1 dependent manner [13–16]. In cancers, MCT1 is preferentially expressed at the plasma membrane of oxidative cancer cells where it facilitates the uptake of lactate together with a proton, thereby alimenting the lactate oxidation pathway and supporting metabolic symbiosis [1]. MCT1 and MCT4 have further been involved in a commensalism behavior of oxidative cancer cells, whereby these cells mobilize and exploit lactate and ketone bodies produced by stromal cells [17–19]. Compared to MCT1 and MCT4, MCT2/SLC16A7 and MCT3/SLC16A8 are less often expressed in cancers [4].

Over the last 8 years, the existence of a metabolic symbiosis has been substantiated in different cancer types, indicating in general terms that this metabolic behavior is an important contributor to tumor progression. Evidence includes the preferential expression of MCT4 in the hypoxic/glycolytic cancer cell compartment and of MCT1 in well-oxygenated tumor areas, as well as the observation that 13C-labelled lactate can be converted into downstream metabolites of the lactate oxidative pathway (such as 13C-alanine) in tumors in vivo [20]. Overall, a metabolic symbiosis has been documented in a variety of human cancers, including head and neck, breast, lung, stomach, colon, bladder, prostate and cervix cancers, as well as gliomas [1, 3, 21–24]. This motivated the development and preclinical evaluation of several MCT inhibitors [25–29], among which AZD3965, initially developed as a mild immunosuppressor [30], is currently evaluated as an anticancer agent in Phase I clinical trials for patients with prostate cancer, gastric cancer or diffuse large B cell lymphoma (ClinicalTrials.gov NCT01791595). The related compound AR-C155858 is a selective MCT1 inhibitor that nevertheless also inhibits MCT2, but only when MCT2 is bound to ancillary protein basigin, whereas its preferred chaperon protein is embigin [31]. In this context, it is therefore of high interest that three independent studies recently assigned to metabolic symbiosis a primary responsibility for the induction of resistance to anti-angiogenic therapies [32–34], thus supporting the use of MCT inhibitors in combination with these treatments.

Although MCT1 inhibitors are being actively developed and AZD3965 recently entered into clinical trials for the treatment of cancer, there is currently no strategy allowing to measure lactate uptake and its inhibition in clinical settings. In this study, we report the original synthesis and preclinical validation of (±)-[18F]-3-fluoro-2-hydroxypropionate ([18F]-FLac) as a tracer of lactate uptake for positron emission tomography (PET). [18F]-FLac was generated in clinical settings and evaluated in the same cancer model that served for the discovery of the metabolic symbiosis of cancers.