Relationship of tafamidis binding site occupancy, transthyretin stabilization, and disease modification in tafamidis treated transthyretin amyloid cardiomyopathy patients

Introduction

Tafamidis inhibits progression of transthyretin (TTR) amyloid cardiomyopathy (ATTR-CM) by binding TTR tetramer and inhibiting its dissociation to monomers which can denature and result in amyloid fibril formation and deposition in cardiac tissue.

Purpose

While the phase 3 ATTR-ACT clinical data clearly demonstrated efficacy, quantification of TTR stabilization in patients and the degree to which the approved dose captures the full potential of the mechanism has yet to be assessed.

Methods

Isothermal titration calorimetry and subunit fraction exchange were used to determine affinities to the two binding sites on TTR. These values were used to develop a model of tafamidis binding to TTR in plasma that was applied to individual patient data to calculate TTR binding site occupancy and the change in total TTR levels induced by TTR stabilization. Population pharmacodynamic (PD) models were developed for three measures of disease progression, plasma NT-proBNP levels, Kansas City Cardiomyopathy Questionnaire – Overall Score (KCCQ-OS), and six-minute walk test (6MWT) distance, to evaluate patient response with the degree of TTR occupancy.

Results

In vitro binding data of wild-type TTR confirmed tafamidis binds two sites of TTR with negative cooperativity and provided precise estimates of the binding affinity to TTR and albumin. Modeling individual patient data of tafamidis exposure and increased TTR plasma levels using the in vitro derived binding affinity values confirms single site binding is consistent with complete tetramer stabilization. Patients given 80 mg tafamidis meglumine, the clinically approved dose for ATTR-CM, had a 92% reduction in unbound, unstabilized TTR, which correlated with a 53% decrease in NT-proBNP elevation, a 56% decrease in KCCQ-OS worsening and a 49% reduced decline in the 6MWT. For 100% receptor occupancy and stabilization, the expected effects on these measures are 58%, 61%, and 54% for NT-proBNP, KCCQ-OS, and 6MWT, respectively.

Conclusions

These results demonstrate a quantitative relationship between TTR stabilization, the mechanism of action of tafamidis, and accepted laboratory and patient-based outcomes in ATTR-CM. These results also support the value of TTR stabilization as a clinically beneficial treatment option which maintains the protein in its physiologically active form within the body.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): This study was sponsored by Pfizer.

Aging infrastructure creates opportunities for cost-efficient restoration of aquatic ecosystem connectivity

A hallmark of industrialization is the construction of dams for water management and roads for transportation, leading to fragmentation of aquatic ecosystems. Many nations are striving to address both maintenance backlogs and mitigation of environmental impacts as their infrastructure ages. Here, we test whether accounting for road repair needs could offer opportunities to boost conservation efficiency by piggybacking connectivity restoration projects on infrastructure maintenance. Using optimization models to align fish passage restoration sites with likely road repair priorities, we find potential increases in conservation return-on-investment ranging from 17% to 25%. Importantly, these gains occur without compromising infrastructure or conservation priorities; simply communicating openly about objectives and candidate sites enables greater accomplishment at current funding levels. Society embraces both reliable roads and thriving fisheries, so overcoming this coordination challenge should be feasible. Given deferred maintenance crises for many types of infrastructure, there could be widespread opportunities to enhance the cost effectiveness of conservation investments by coordinating with infrastructure renewal efforts.

Minimizing opportunity costs to aquatic connectivity restoration while controlling an invasive species

Controlling invasive species is critical for conservation but can have unintended consequences for native species and divert resources away from other efforts. This dilemma occurs on a grand scale in the North American Great Lakes, where dams and culverts block tributary access to habitat of desirable fish species and are a lynchpin of long-standing efforts to limit ecological damage inflicted by the invasive, parasitic sea lamprey (Petromyzon marinus). Habitat restoration and sea-lamprey control create conflicting goals for managing aging infrastructure. We used optimization to minimize opportunity costs of habitat gains for 37 desirable migratory fishes that arose from restricting sea lamprey access (0-25% increase) when selecting barriers for removal under a limited budget (US$1-105 million). Imposing limits on sea lamprey habitat reduced gains in tributary access for desirable species by 15-50% relative to an unconstrained scenario. Additional investment to offset the effect of limiting sea-lamprey access resulted in high opportunity costs for 30 of 37 species (e.g., an additional US$20-80 million for lake sturgeon [Acipenser fulvescens]) and often required ≥5% increase in sea-lamprey access to identify barrier-removal solutions adhering to the budget and limiting access. Narrowly distributed species exhibited the highest opportunity costs but benefited more at less cost when small increases in sea-lamprey access were allowed. Our results illustrate the value of optimization in limiting opportunity costs when balancing invasion control against restoration benefits for diverse desirable species. Such trade-off analyses are essential to the restoration of connectivity within fragmented rivers without unleashing invaders.

Conserving rare species can have high opportunity costs for common species

Conservation practitioners face difficult choices in apportioning limited resources between rare species (to ensure their existence) and common species (to ensure their abundance and ecosystem contributions). We quantified the opportunity costs of conserving rare species of migratory fishes in the context of removing dams and retrofitting road culverts across 1,883 tributaries of the North American Great Lakes. Our optimization models show that maximizing total habitat gains across species can be very efficient in terms of benefits achieved per dollar spent, but disproportionately benefits common species. Conservation approaches that target rare species, or that ensure some benefits for every species (i.e., complementarity) enable strategic allocation of resources among species but reduce aggregate habitat gains. Thus, small habitat gains for the rarest species necessarily come at the expense of more than 20 times as much habitat for common ones. These opportunity costs are likely to occur in many ecosystems because range limits and conservation costs often vary widely among species. Given that common species worldwide are declining more rapidly than rare ones within major taxa, our findings provide incentive for triage among multiple worthy conservation targets.