A Study to Learn More About the Change in the Blood Levels of Transthyretin When Participants With Transthyretin Amyloidosis With Cardiomyopathy Switch From Tafamidis to Acoramidis

Transthyretin (TTR) is a protein made by the liver that helps transport thyroid hormone and vitamin A in the blood. In some people, this protein breaks down and forms harmful clumps called amyloid. TTR amyloid gets deposited in the heart wall and stops it from pumping blood properly, which may lead to heart failure. The breakage in TTR protein can be age-related (wild-type ATTR-CM), or genetic (variant ATTR-CM). The study drug, acoramidis, works by attaching itself to the TTR protein, making TTR more stable and less likely to break down and form amyloid (clumps). This helps to slow down the progression of the disease, improve heart function, and increase the TTR levels in the blood. Acoramidis is an approved treatment for wild-type or variant ATTR-CM in Europe and the United States. Tafamidis is another drug that stabilizes TTR and prevents it from breaking down. In this study, acoramidis will be studied in participants with ATTR--CM who were previously treated with tafamidis. The main purpose of this study is to assess the change in blood TTR levels after participants are switched from tafamidis to acoramidis. This will be studied to understand if acoramidis causes an increase in blood TTR levels beyond the levels achieved with tafamidis. For this, the researchers will measure the change in the levels of TTR protein in participants' blood after 6 months of the treatment with acoramidis, or earlier if a participant stops the treatment before reaching that six-month mark. All participants will continue taking tafamidis during the screening period. In the treatment period of the study, participants will take acoramidis as two tablets twice daily by mouth, for up to 6 months. At the start of this study, the study doctors will review each participant's medical history and check their overall health. The study doctors will perform electrocardiograms (ECG), and measure blood pressure and heart rate. Researchers will also take blood and urine samples from the participants to measure levels of TTr, NT-proBNP, hs-TnT, hs-CRP, RBP4, eGFR, creatinine, cystatin-C, UACR, and TSH at the start of the study, and at various time points thereafter (during the study) to assess heart, kidney and thyroid function. There will be a total of 9 study check-ins. Participants will visit the study site twice: at screening and at the end of treatment period. A study nurse will visit the participant's home 6 times, at the start of treatment, Weeks 1, 2, 3 and 4, then again at 3 months. The final check-in will be done by phone. The study doctors will monitor the health of the participants regularly for any medical problems during follow-up visits. Participants will know the treatment they will receive during the study. Each participant could be in the study for about 8 months.