Enabling Noninvasive Visualization of AT1R Through a New ¹⁸F PET Radiotracer

The renin–angiotensin system (RAS) is a cornerstone of cardiovascular and renal physiology, orchestrating the delicate equilibrium between vasoconstriction, sodium balance, and tissue remodeling. At its center stands the angiotensin II type 1 receptor (AT1R), a G protein-coupled receptor mediating most of angiotensin II’s canonical effects: vasoconstriction, aldosterone release, and pro-fibrotic signaling.¹ Overactivation of AT1R has been implicated in hypertension, chronic kidney disease, heart failure, and in fibrotic and inflammatory remodeling of other organs.² Despite the therapeutic success of AT1R blockers (“sartans”) and ACE inhibitors,³ our ability to noninvasively visualize and quantify AT1R expression in vivo has remained limited, leaving a significant gap between molecular pathophysiology and patient-level diagnostics.

Before [¹⁸F]DR29, AT1R PET imaging faced several obstacles. Carbon-11–labeled tracers such as [¹¹C]KR31173 demonstrated feasibility in pigs and baboons, but clinical translation was limited by production logistics and rapid isotope decay.⁴ Fluorine-18–labeled derivatives showed promise, yet either lacked sufficient renal retention or suffered from rapid hepatobiliary clearance, limiting image contrast. Furthermore, the influence of OATs and OATPs on biodistribution was appreciated but not systematically addressed. Importantly, the kidney’s physiologic expression of AT1R posed both an opportunity—to study disease-related upregulation—and a challenge—to separate receptor-specific uptake from transporter-mediated confounders.

Molecular imaging, particularly with positron emission tomography (PET), offers a potential bridge across this gap by allowing the spatial and temporal quantification of receptor expression, pharmacodynamics, and tissue remodeling.⁵ Historically, the field has relied on carbon-11–labeled tracers such as [¹¹C]KR31173, which demonstrated encouraging preclinical performance but were constrained by the 20-minute half-life of carbon-11, limiting clinical utility to specialized centers with on-site cyclotrons. Recognizing this limitation, several groups sought fluorine-18–labeled alternatives, leveraging the longer half-life (110 minutes) to permit broader distribution and more extensive imaging protocols (Table 1). Among these options, [¹⁸F]FPyKYNE-losartan and derivatives of irbesartan ([¹⁸F]FV45) showed that fluorine-18 labeling could preserve AT1R affinity while enabling high-yield radiolabeling and favorable in vivo stability.^{6, 7} Yet these agents still faced hurdles, including suboptimal kidney-to-liver contrast, rapid washout, or high nonspecific uptake driven by transporters like organic anion transporting polypeptides (OATPs) and organic anion transporters (OATs); these membrane transporters, though physiologically essential, can sequester tracers in liver or renal tubular cells, obscuring receptor-specific binding.⁸

Table 1.

Main characteristics of selected PET tracers for AT1R imaging.

Tracer	Isotope	Half-life	Design / Chemical strategy	Features
[11C]KR31173	Carbon-11	~20’	Carbon-11 labeling on KR31173 scaffold	High affinity; early proof-of-concept in pigs/baboons; receptor specificity validated by blocking; requires on-site cyclotron; short half-life;
[18F]FPyKYNE-losartan	Fluorine-18	~110’	Fluorine-18 prosthetic group conjugated to losartan derivative	Longer half-life allows wider distribution; preserves AT1R affinity; suboptimal kidney-to-liver contrast; rapid washout; transporter-mediated nonspecific uptake
[18F]FV45	Fluorine-18	~110’	Fluorine-18 labeled derivative of irbesartan	Good AT1R affinity; high radiochemical yield; rapid hepatobiliary clearance; lower renal retention; influence of organic anion transporting polypeptides not fully addressed
[18F]DR29	Fluorine-18	~110’	Fluorine-18 at α-position on SK1080 scaffold	High affinity (subnanomolar); high radiochemical purity; transporter blocking strategy reduces nonspecific uptake; validated in hypertensive rats; good renal retention

In this context, the study by Chen and collaborators in the current issue of Hypertension, introducing [¹⁸F]DR29, is both timely and technically innovative.⁹ They present a carefully executed, conceptually sophisticated study introducing [¹⁸F]DR29 as a promising fluorine-18–labeled PET tracer for AT1R imaging. By designing a molecule with high affinity and stability, and systematically addressing transporter-mediated confounders, they achieve clear, receptor-specific renal images, validated in a hypertensive disease model. The authors built on the high-affinity scaffold SK1080, integrating a fluorine-18 at the α-position of the aliphatic chain, a strategy informed by prior experience suggesting preservation of affinity and metabolic stability. Their approach is commendable for several reasons. First, it addresses a clear unmet need: imaging AT1R expression in the kidney, where high physiologic AT1R density—and pathologic upregulation in hypertension—offers both a challenge and an opportunity for molecular imaging. Second, it systematically tackles the longstanding confounding factor of transporter-mediated nonspecific uptake by applying pharmacological blockade with rifampicin (OATP inhibitor) and nitazoxanide (OAT inhibitor). The methodological rigor is evident. Radiolabeling achieved a decay-corrected yield of ~36%, with high radiochemical purity (>99%), and PET imaging protocols were carefully designed to explore tracer kinetics, specificity, and organ distribution. The use of candesartan, an AT1R blocker, provided pharmacologic validation of target specificity, showing markedly reduced renal uptake after pretreatment.

Perhaps most compelling is the study’s translational relevance: in hypertensive rats, [¹⁸F]DR29 displayed higher renal uptake than in normotensive controls, paralleled by increased AT1R expression on immunoblots and immunohistochemistry. This consistency across modalities strengthens confidence in the biological specificity of the tracer.

[¹⁸F]DR29 represents a new fluorine-18–labeled AT1R PET tracer that achieves several advances, including a rational design strategy that preserves subnanomolar AT1R affinity, a high radiochemical purity and reasonable radiochemical yield, making it feasible for broader use, a careful application of transporter blockers to minimize nonspecific hepatic and renal tubular uptake (thereby clarifying receptor-mediated binding), a demonstration of higher renal uptake in hypertensive rats, validated by ex vivo protein expression data. Beyond the technical achievement, a conceptual advance also transpires: by combining PET imaging with transporter pharmacology, AT1R-specific signals are isolated and highlighted, setting a new standard for molecular imaging of GPCRs in organs where transporter expression is high.

As with any preclinical study, several limitations warrant discussion. First, the sample sizes, while adequate for proof-of-concept, are small, particularly in PET imaging (n=2 per group). Larger cohorts would strengthen statistical power and allow for better characterization of inter-individual variability. Second, species differences in transporter expression and AT1R distribution complicate direct translation to humans. Rodents, for instance, lack OAT4, which is present in humans and plays a role in renal excretion of sartans. Similarly, OATP subtypes differ in expression patterns across species, potentially altering tracer pharmacokinetics. Third, the study used male rats exclusively; sex differences in transporter expression (e.g., higher OAT1 in males, higher OAT2 in females) may affect tracer distribution and should be explored.

Moreover, while the blocking strategy (rifampicin and nitazoxanide) effectively reduced nonspecific uptake, these drugs are not standard in clinical imaging protocols and may introduce variability or side effects in human studies. Their translation would require safety testing, and the balance between improved specificity and added complexity would need evaluation.

Another consideration is the molar activity, which, while sufficient for small animal imaging, was relatively low (1.9–5.1 GBq/μmol). Higher molar activity would reduce potential saturation of receptor binding sites and improve sensitivity, especially critical when imaging low-density targets or small changes over time. Finally, the study focused primarily on kidneys, where high AT1R density ensures robust signal; whether [¹⁸F]DR29 can image AT1R expression in organs with lower receptor density—such as myocardium or vasculature—remains to be shown. Given prior work demonstrating upregulation of AT1R in post-ischemic myocardium, this is an exciting and logical next step¹⁰.

The implications of this work are broad and significant. In nephrology, [¹⁸F]DR29 could provide a noninvasive readout of renal AT1R expression, offering insights into disease activity in hypertension, diabetic nephropathy, or chronic kidney disease. Quantifying AT1R may guide selection and titration of sartans, moving from empirical dosing to receptor-guided therapy—a step toward precision medicine. In cardiology, imaging AT1R in the heart could help identify patients at risk for maladaptive remodeling after myocardial infarction, potentially informing early intervention. Previous work with [¹¹C]KR31173 showed increased cardiac AT1R binding after ischemia/reperfusion; a fluorine-18 tracer could expand this paradigm to clinical PET centers lacking an on-site cyclotron. Further, AT1R quantification can potentially aid in distinguishing nonclassical mechanisms of hypertension such as the ACE2/Ang1-7 pathway, leading to effective personalized treatment regimens.

Beyond hypertension and heart disease, AT1R is increasingly recognized in fibrosis, inflammation, and oncology.¹¹ Tumors overexpressing AT1R might be visualized and characterized with [¹⁸F]DR29, opening exploratory avenues in oncology and immunology. Another intriguing prospect lies in drug development. PET imaging with [¹⁸F]DR29 could quantify receptor occupancy by new AT1R antagonists, accelerating pharmacodynamic studies and enabling dose optimization. Such applications mirror the success of PET tracers in neuropharmacology, where receptor occupancy studies have guided antipsychotic and antidepressant dosing. Lastly, the methodology—pairing transporter inhibition with molecular imaging—could be generalized to other tracers and targets. Many radiopharmaceuticals face nonspecific uptake mediated by transporters; understanding and modulating these pathways can refine image specificity and quantitative accuracy.

This work exemplifies how rational radiotracer design, informed by pharmacology and disease biology, can move molecular imaging closer to precision medicine. While several translational hurdles remain—including species differences, blocking strategies, and molar activity—the potential clinical applications are compelling, spanning nephrology, cardiology, and beyond.

The next steps—larger animal models, dosimetry studies, and ultimately, first-in-human trials—will determine whether [¹⁸F]DR29 fulfills its promise. If successful, it could become an invaluable tool to visualize, quantify, and personalize therapy targeting one of the most important receptors in cardiovascular and renal disease.