Background: Systemic lupus erythematosus (SLE) is a systemic autoimmune disease in need of better treatments^1–6. SLE affects up to 4 million people worldwide and presents heterogeneously with symptoms that can range from fever, rash, and alopecia to – in rare cases – myocardial infarction and stroke. Lupus nephritis, caused by pathology in the kidneys, occurs in up to 50% of patients and can lead to end-stage renal disease. Other sites of pathology include major organs such as the skin, heart, and lungs. Like many autoimmune diseases, SLE is caused by CD4 T and B cells that are aberrantly activated against self-antigens and that require rapid proliferation to yield sufficient cells to cause damage. Only three therapies are approved for SLE, and most standard-of-care therapies are strongly immunosuppressive. These therapies can be effective, but response rates remain low (<50%), drug efficacy can wane over time, and side-effects can be severe including increased risk of serious infections and drug-induced organ damage. New therapies are needed.

The deoxyribonucleoside salvage pathway with rate-limiting enzyme deoxycytidine kinase (dCK) salvages extracellular deoxyribonucleosides including deoxycytidine for intracellular deoxyribonucleotide triphosphate biosynthesis and DNA replication⁷. Deoxyribonucleoside salvage and dCK activity can be quantified in vivo with PET using the fluorinated deoxycytidine analogue [¹⁸F]FAC in mice and [¹⁸F]clofarabine in humans^8,9. Previous studies imaging mouse models of autoimmune hepatitis and multiple sclerosis with [¹⁸F]FAC PET demonstrated that dCK is activated in the lymphoid organs in these models^10–12. However, the role of dCK in SLE remains understudied. TRE-515 is a small molecule inhibitor of dCK in Phase 1 clinical trials for solid tumors that has previously been shown to treat disease in mouse multiple sclerosis models¹², suggesting important clinical relevance and therapeutic potential to studying dCK activity in SLE.

Hypothesis: We hypothesize that dCK activity is elevated during and necessary for SLE in a mouse model.

Methods: We studied the MRL-Fas^lpr mouse model of SLE, which manifests with SLE phenotypes including proteinuria, elevated serum IgG and anti-double stranded DNA antibodies, and kidney inflammation^13–16. We imaged MRL control and MRL-Fas^lpr SLE mice with [¹⁸F]FAC PET/CT throughout the disease course starting at the presymptomatic phase (6-weeks-old) through mild (10-weeks-old), moderate (14-weeks-old), and severe (18-weeks-old) disease. We measured disease phenotypes in the imaged mice. Blocking studies with excess deoxycytidine were used to evaluate specificity of tracer accumulation. Results were analyzed to evaluate dCK activity in various organs and to determine whether [¹⁸F]FAC accumulation in these organs correlated with disease phenotypes. Ex vivo studies to validate the imaging results and treatment studies with TRE-515 are ongoing.

Results: dCK activity, as measured by [¹⁸F]FAC accumulation, was significantly elevated specifically in the lymph nodes of the MRL-Fas^lpr SLE mice at 10, 14, and 18 weeks of age compared to 10 week-old-control MRL mice (14 and 18 week MRL mice have been out of stock but will be studied soon). Excess deoxycytidine blocked [¹⁸F]FAC accumulation in the lymph nodes, demonstrating specificity. dCK activity measured by [¹⁸F]FAC accumulation was unaffected by disease and generally low in other organs including the heart, brain, and lungs. dCK activity in the lymph nodes correlated with measures of disease activity including serum IgG and proteinuria levels. These results suggest that elevated dCK activity may be necessary for SLE in this model, that the likely site-of-action for dCK activity in this model is the lymph nodes, and that targeting dCK in this model should have limited on-target effects outside of the lymphoid organs.

Conclusions: dCK activity is elevated in the lymph nodes during disease and correlates with disease phenotypes in a mouse SLE model, suggesting dCK as a potential target for treating SLE.

Image/Figure:

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Image/Figure Caption:

Deoxycytidine kinase activity, as measured by [¹⁸F]FAC PET, is upregulated specifically in the lymph nodes in the MRL-Fas^lpr mouse model of systemic lupus erythematosus. 

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