Introduction: Cancer cachexia (CC) is a debilitating wasting condition defined as significant loss of muscle mass with or without fat loss. While anorexia and muscle wasting are features of cachexia, CC is considered a distinct metabolic syndrome, driven by tumours and their interaction with the host¹, however these multi-organ whole-body metabolic changes remain poorly understood. We aimed to understand how glucose uptake, measured on pre-clinical [¹⁸F]fluorodeoxyglucose (FDG) PET imaging, changes during lung CC and how this compares to conditions of anorexia and muscle wasting.

Methods: Genetically-engineered mouse models of lung CC, K-ras^G12D/+;Lkb1^-/-(KL) were induced via intranasal administration of Ad5CMVCre virus 70µL and titre 2.5 × 10⁷ pfu. KL mice were imaged alongside K-ras wild-type (WT) non-tumour bearing controls, once KL mice reached 15-19% body weight loss (Fig 1A).

In non-tumour bearing male animals, we modelled anorexia, muscle wasting and circulation of cachexia factor GDF15 as follows, respectively: fasted overnight for 20 hr, treated with dexamethasone 21-phosphate (dexa) (2mg/kg, i.p. daily, 21 days) and single injection of recombinant human GDF15 hormone (0.1 mg/kg s.c.).

For imaging, mice were injected with 12-18MBq of FDG, imaging 80-100 minutes post injection with the Mediso nanoScan® PET/MRI 1T. For studies with GDF15 and dexa treatment, animals remained under anaesthesia during uptake period. Frozen samples of serum and tissue were collected for ex vivo metabolomics. Organ segmentation of co-registered MRI images was completed manually, and activity per organ quantified. For fasting and cachexia studies, FDG uptake was conscious and post imaging, fresh blood and tissue samples were counted in the HIDEX automatic gamma counter for ex vivo biodistribution analysis.

Results/Discussion:

In the KL CC model, FDG imaging was conducted upon 15-19% body weight loss (Fig 1A). Endogenous blood glucose decreased in KL mice (20.5 mmol/L WT vs. 8.5 mmol/L KL) mice (Fig 1B). FDG ex vivo biodistribution showed increased uptake in myocardium (p<0.05), brain and liver (both p<0.01) compared to WT animals (Fig 1C,D), however there was no significant change in FDG blood pool (Fig 1D).  

Fasting for 20 hr resulted in an average 12% body weight decrease and decreased endogenous blood glucose (14 mmol/L fed vs.4.8 mmol/L fasted animals, Fig 2 A,B). Like cachexic KL animals, FDG uptake was increased in brain (p<0.0001) and several visceral organs including liver (p<0.05), likely due to increased FDG blood pool (p<0.01). FDG uptake in limb muscles and brown adipose tissue decreased (both p<0.05) suggesting lower muscle activity and decreased thermogenesis respectively.

We previously found dexa treatment to increase E3 ligase Murf1 expression in gastrocnemius muscle after 6 weeks (2mg/kg daily i.p, Fig 3A). On imaging, dexa treatment decreased FDG uptake in gastrocnemius/soleus muscles 3 weeks post treatment (SUV_mean 0.63 vehicle vs. 0.45 dexa, p<0.05, Fig 3B), accompanied by a 5-fold decrease in glucose-6 phosphate in gastrocnemius muscle on ex vivo metabolomics (Fig 3C), suggesting alterations in glucose uptake during muscle wasting.

Injection of cachexia factor GDF15 increased circulating levels from non-detectable in control to 7288 ± 1916 pg/ml in treated animals (p<0.001, Fig 4A) at 4 hours post administration, accompanied by a significant increase in FDG uptake in quadricep muscle (p<0.05, Fig 4B) and aortic blood (p<0.01, Fig 4B). In contrast to KL animals, GDF15 injection resulted in decreased FDG uptake in myocardium (p<0.05, Fig 4B). Alterations in glycolysis in the myocardium was corroborated with ex vivo metabolomics, with a 2-fold increase in glucose-6 phosphate in heart tissue compared to saline treated animals (Fig 4C).

Conclusion: We compared FDG PET signatures in lung CC, anorexia and muscle wasting. In future we aim to understand the biological underpinning of these signatures and their relevance to clinical CC phenotypes.  

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Image/Figure Caption:

Figure 1. FDG metabolic PET phenotype in lung cancer cachexia KL mouse model. A) Weight trajectories in KL (pink) and non-tumour bearing controls (WT, black). Imaging conducted at end-point, 15-19% body weight loss. Each line corresponds to one animal. B) Blood glucose mmol/L measured by ACCU-CHEK® from cardiac blood at end-point. C) Exemplar maximum intensity projection of 18-F FDG uptake across WT and KL animals. Major organs of interest are labelled as follows B=brain, H=heart, L=lung tumour, M=muscles, lower limb. D) Ex vivo biodistribution of FDG measured in %injected dose (ID) per gram of tissue. In B and D each dot corresponds to data from one animal. *p<0.05, **p<0.01.

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