A positron emission tomography image-guidable unimolecular micelle nanoplatform for cancer theranostic applications

Abstract Unimolecular micelles based on hyperbranched polyamidoamine (PAMAM) dendrimer were synthesized as both a cargo delivery vector and an imaging agent for triple-negative breast tumors, and the chemical synthesis procedures are detailed in this study. With the chemical conjugation of a peptide (F3, against cellular nucleolin) to increase its cellular internalization, these micelles can accumulate potently and specifically in breast cancer cells (e.g. , MDA-MB-231). The size and morphology of these PAMAM-based micelles have been measured by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The hydrazone bond (responsive to pH alteration) between the loaded doxorubicin (DOX, as a model drug here) and PAMAM micelles enables cargo release following pH changes. Flow cytometry and confocal fluorescence microscopy revealed that PAMAM micelles with F3 attachment (PAMAM-DOX-F3) had stronger internalization into MDA-MB-231 cells (nucleolin-positive) than PAMAM micelles without F3 conjugation (PAMAM-DOX), whereas both of them have minimal interactions with L929 fibroblasts (nucleolin-negative). The positron-emitting isotope 64 Cu was added into PAMAM micelles by chelation to track their pharmacokinetic behavior (organ distribution profile) in vivo by positron emission tomography (PET) imaging. Serial PET imaging demonstrated that the accumulation of 64 Cu-PAMAM-DOX-F3 in MDA-MB-231 tumors was fast, potent, and persistent (tumor uptake: 6.1 ± 1.2% injection dose per gram [%ID/g] at 24 h p.i .), significantly higher than that of 64 Cu-PAMAM-DOX (2.5 ± 0.4%ID/g at the same time). Their distribution profiles in other organs/tissues were quite similar, with a relatively short circulation time. In addition, ex vivo fluorescence imaging confirmed that DOX can be delivered efficiently by these PAMAM micelles to MDA-MB-231 tumors. Deducing from these data, we believe that PAMAM-based micelles can be useful for selective combinational treatment of cancer. Statement of Significance Micelles are a very useful biomaterial for theranostic purposes, and one of the major hurdles for micelles (particularly those from self-assembling) is their relatively low stability, especially when administered in vivo. In this study, we have attempted to overcome this limitation by designing unimolecular micelles (based on the concept of “one micelle is composed of one macromolecule”) from polyamidoamine (PAMAM) dendrimers, in which the drug cargos (e.g., doxorubicin) are chemically attached to PAMAM through a hydrazone bond; hence, they can be used as a tumor-selective diagnostic/therapeutic platform. These unimolecular micelles possess superior stability compared to conventional micelles and can undertake stimulus (pH)-responsive cargo release for more “targeted” cancer therapy. With the incorporation of a tumor-targeting peptide sequence (F3) and a positron-emitting isotope (copper-64), the pharmacokinetic behavior of these micelles can be readily monitored by positron emission tomography imaging technique to confirm their specificity against cancer tissues. With further optimization, this micellar platform can have a broad clinical applicability owing to its biocompatibility, selectivity, and stability.