Mechanism of Self-Assembly of the Gonadropin Releasing Hormone Antagonist Teverelix into Amyloid Fibrils

TL;DR

This paper studies how teverelix, a cancer treatment, forms amyloid-like fibrils, which may explain its long-lasting effect in the body.

Contribution

The first detailed biophysical study of teverelix fibril formation and its mechanism.

Findings

Teverelix forms amyloid-like fibrils with a larger inter-β-sheet packing distance due to non-natural side chains.

Fibrillation rate depends on pH, peptide concentration, and trifluoroacetic acid levels.

A nucleation–polymerization mechanism is proposed for teverelix fibril formation.

Abstract

Teverelix is a short non-natural peptide, which is a
gonadotropin
releasing hormone antagonist and used as a treatment for prostate
cancer. Teverelix is formulated as a trifluoroacetic acid salt, which,
at the high concentrations used for parenteral injection, forms a
microcrystalline suspension. At low concentrations and immediately
after injection, teverelix self-assembles into a fibrillar species
thought to be important for the slow-release kinetics and long-acting
action of this peptide in vivo. In this paper, we confirmed the amyloid-like
identity of teverelix fibrils using X-ray fiber diffraction and transmission
electron microscopy. The inter-β-sheet packing distance was
found to be larger than that of typical amyloid fibrils and this was
attributed to the large non-natural side chains within the peptide.
Using data from numerous biophysical experiments, a model of the structure
of teverelix within the fibril is proposed. The kinetics of fibril
formation were investigated using standard ThT assays, and teverelix
found to fibrillate rapidly over a wide range of conditions. The fibrillation
rate was shown to depend critically upon pH, peptide, and trifluoroacetic
acid concentration. Fibrillation was accompanied by a drop in pH,
which we attribute to the fact that the pyridinium side chain must
be deprotonated before self-assembly. Based on our results, we propose
a nucleation–polymerization mechanism in which dimers of teverelix
rapidly self-assemble into amyloid-like fibrils with little change
in the secondary structure but burial of some of the aromatic acid
side chains. Interestingly, the fibrils can, under certain conditions,
align to create a highly ordered array. To the best of our knowledge,
this is the first paper studying teverelix in detail from a biophysical
perspective, and it is directly relevant to the aggregation of the
peptide observed in vivo.

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