Kinetic quantification of protein polymer nanoparticles using non-invasive imaging

Abstract

Protein polymers are repetitive amino acid sequences that can assemble monodisperse nanoparticles with potential applications as cancer nanomedicines. Of the currently available molecular imaging methods, positron emission tomography (PET) is the most sensitive and quantitative; therefore, this work explores microPET imaging to track protein polymer nanoparticles over several days. To achieve reliable imaging, the polypeptides were modified by site-specific conjugation using a heterobifunctional sarcophagine chelator, AmBaSar, which was subsequently complexed with 64Cu. AmBaSar/64Cu was selected because it can label particles in vivo over periods of days, which is consistent with the timescales required to follow long-circulating nanotherapeutics. Using an orthotopic model of breast cancer, we observed four elastin-like polypeptides (ELPs)-based protein polymers of varying molecular weight, amino acid sequence, and nanostructure. To analyze this data, we developed a six-compartment image-driven pharmacokinetic model capable of describing their distribution within individual subjects. Surprisingly, the assembly of an ELP block copolymer (78 kD) into nanoparticles (Rh = 37.5 nm) minimally influences pharmacokinetics or tumor accumulation compared to a free ELP of similar length (74 kD). Instead, ELP molecular weight is the most important factor controlling the fate of these polymers, whereby long ELPs (74 kD) have a heart activity half-life of 8.7 hours and short ELPs (37 kD) have a half-life of 2.1 hrs. These results suggest that ELP-based protein polymers may be a viable platform for the development of multifunctional therapeutic nanoparticles that can be imaged using clinical PET scanners.

Controlled release from recombinant polymers

“…This result may show the micelles formed could be unstable in blood and may break into unimer strands, which would show the nearly identical pharmacokinetic parameters observed. [58]…”

Elastin-like polypeptides: Therapeutic applications for an emerging class of nanomedicines

“…Considering that polymer molecular weight plays a large role with regard to in vivo drug carrier disposition, ELP monodispersity lends itself to the control of pharmacokinetic parameters such as biodistribution and clearance rate. Other studies were integral in establishing the foundations for using microPET imaging as a non-invasive means of tracking ELP biodistribution and image-driven pharmacokinetics[82]. This was accomplished by conjugating a chelating agent, AmBaSar, onto the ELP prior to radiolabeling with 64 Cu.…”

“…As a biotherapeutic, ELPs have agreeable pharmacokinetics with terminal circulation half-lives ranging from 8 to 12 hours in mice[12, 82, 84]. Based on prior work in small animal models, ELPs also exhibit biocompatibility and low immunogenicity[85-87].…”

Genetically engineered nanocarriers for drug delivery

“…Depending on molecular weight and assembled structure, the blood half-lives of ELPs vary from 2 to 6 hours in vivo ( Figure 3E). 23 Because the half-life of Rapa release is much longer than the blood half-lives of ELPs, it has been speculated that the drug will remain associated with the carrier blood circulation with minimal detachment, which may reduce systemic side effects.…”

“…4 These block copolymers have been verified to form stable nanoparticle structures ranging from 50-90 nm in diameter, which have various functions in drug delivery, and the formation of which is dependent on the difference between the transition properties of the hydrophilic and hydrophobic blocks. 2,6,16,18,23,[30][31][32] Figure 2 illustrates a series of ELP micelle nanoparticles formed by repetitive amino-acid sequences with different guest residues in hydrophobic and hydrophilic blocks. 4,23,33,34 …”