Non-invasive imaging of fibrosis with positron emission tomography in a rat model with systemic hypertension and myocardial fibrosis
Heart failure is one of the leading causes of death worldwide. Hypertension can initiate myocardial remodelling processes which, often via fibrotic triggers through the renin-angiotensin-aldosterone system, can lead to the development of heart failure. A main contributor of these pathways is angiotensin II, increased levels of which can induce volume and pressure overload in the cardiovascular system, making it an important factor in both hypertension and associated cardiovascular disease. A main process during cardiac remodelling is fibrosis which can be divided into two types: reactive and replacement fibrosis. The latter refers to the changes via scar formation at an injury site while the former (interstitial or perivascular fibrosis) can happen as a response to changes in the physical or chemical environment within the tissue such as hypertension or inflammation. Fibrillary collagen is an important extracellular matrix component and abundantly deposited during fibrosis. Collagen can have various subtypes based on its structure which can add different characteristics to the tissue. During collagen biosynthesis, cis- or trans-proline containing pro-α chains can be integrated into the protein, where chains containing cis isomer are associated with more distensible and abnormal collagen and those with trans isomer with more rigid triple helix collagen. Other factors can also influence the development of heart failure via the myocardial remodelling processes, such as inflammatory and angiogenic pathways. Heart failure can be diagnosed and assessed in the clinic via blood tests and imaging techniques such as ultrasound, magnetic resonance imaging (MRI), computerised tomography (CT), and single-photon emission computed tomography (SPECT) / positron emission tomography (PET). This thesis aimed to investigate the effect of increased angiotensin II and subsequent hypertension on the levels of myocardial collagen synthesis and to test PET radiotracers cis-4-18F-fluoro-L-proline and trans-4-18F-fluoro-L-proline for the detection of myocardial fibrosis and potential differentiation of the types of collagen fibers. The overarching hypothesis of the project was that myocardial fibrosis can be imaged non-invasively with PET in a rat pressure overload model with via persistent hypertension resulting in end-organ damage. A hypertensive rat model with myocardial remodelling was established via angiotensin II infusion using osmotic mini-pumps. Treatment length and dosage were tested and the optimal protocol was chosen to induce myocardial fibrosis. The model was assessed for myocardial collagen content as well as markers of inflammation and vasculature. On a separate set of experiments, the performance of PET radiotracers, cis-4-18F-fluoro-L-proline and trans-4-18F-fluoro-L-proline, was assessed in naïve rats to understand their in vivo metabolism and kinetics. Then, the optimised animal model of hypertensive heart failure and PET imaging protocols were used to investigate whether the new imaging probes could visualise areas if increased collagen synthesis and whether the uptake was related to the type of collagen involved. Using 500 ng/kg/min angiotensin II dose for 4 weeks duration was adequate to induce myocardial fibrosis and hypertension in the rat model. The fibrosis pattern was mainly perivascular in nature. Immunostaining also showed increased CD68 in the myocardium of rats on 250 ng/kg/min but not with the higher dose. The highest percentage of cells stained positive for all three of CD68, TSPO and isolectin B4 was found in the atria of animals on the higher angiotensin II dose, which area also showed the most fibrosis overall. Both radiotracers were successfully assessed in naïve rats, showing no metabolism and favourable kinetics in vivo, allowing for simplified quantification of radiotracer uptake. Myocardial radiotracer uptake in the angiotensin II treated cohort showed increased myocardial signal with the trans-4-18F-fluoro-L-proline radiotracer but no significant differences with cis-4-18F-fluoro-L-proline compared to vehicle treated animals. Animals undergoing the imaging experiment showed increased myocardial fibrosis similarly to rats in the model development experiments. Myocardial fibrosis develops during hypertension induced via angiotensin II treatment, and this change can be measured with PET imaging targeting collagen biosynthesis. Further investigation into the types of collagens involved and how they contribute to pathology needs to be carried out to characterise the underlying biological processes in more detail. Fluoroproline radiotracer PET imaging can become a valuable tool for the assessment of fibrosis pathology both in terms of early detection and disease progression.