Ligand-targeted theranostic nanomedicines against cancer

Abstract

Nanomedicines have significant potential for cancer treatment. Although the majority of nanomedicines currently tested in clinical trials utilize simple, biocompatible liposome-based nanocarriers, their widespread use is limited by non-specificity and low target site concentration and thus, do not provide a substantial clinical advantage over conventional, systemic chemotherapy. In the past 20years, we have identified specific receptors expressed on the surfaces of tumor endothelial and perivascular cells, tumor cells, the extracellular matrix and stromal cells using combinatorial peptide libraries displayed on bacteriophage. These studies corroborate the notion that unique receptor proteins such as IL-11Rα, GRP78, EphA5, among others, are differentially overexpressed in tumors and present opportunities to deliver tumor-specific therapeutic drugs. By using peptides that bind to tumor-specific cell-surface receptors, therapeutic agents such as apoptotic peptides, suicide genes, imaging dyes or chemotherapeutics can be precisely and systemically delivered to reduce tumor growth in vivo, without harming healthy cells. Given the clinical applicability of peptide-based therapeutics, targeted delivery of nanocarriers loaded with therapeutic cargos seems plausible. We propose a modular design of a functionalized protocell in which a tumor-targeting moiety, such as a peptide or recombinant human antibody single chain variable fragment (scFv), is conjugated to a lipid bilayer surrounding a silica-based nanocarrier core containing a protected therapeutic cargo. The functionalized protocell can be tailored to a specific cancer subtype and treatment regimen by exchanging the tumor-targeting moiety and/or therapeutic cargo or used in combination to create unique, theranostic agents. In this review, we summarize the identification of tumor-specific receptors through combinatorial phage display technology and the use of antibody display selection to identify recombinant human scFvs against these tumor-specific receptors. We compare the characteristics of different types of simple and complex nanocarriers, and discuss potential types of therapeutic cargos and conjugation strategies. The modular design of functionalized protocells may improve the efficacy and safety of nanomedicines for future cancer therapy.

Keywords: Antibody display; Peptide ligands; Phage display; Protocells; Tumor targeting.

Fig. 1

Drug development pipeline for BMTP-11. Development of a peptide-based therapeutic, BMTP-11, starting from in vivo phage display using a peptide combinatorial library in a terminal wean patient identified a prostate tumor-specific peptide ligand, which was followed by receptor identification and validation. Drug development of BMTP-11 included toxicological studies in mice and cynomolgus monkeys followed by a first-in-man phase 0 clinical trial, in which BMTP-11 localized and induced apoptosis of tumor cells at a secondary metastatic site [23].

Fig. 2

Comparison of simple vs. complex nanocarriers. Complex nanocarriers incorporate high loading capacity of a variety of cargos, greater stability and high biocompatibility. The lipid bilayer of the functionalized protocell may contain selective polymers, such as PEG (green) or cholesterol (purple diamonds) to improve membrane fluidity and overall charge. Additional functional moieties for conjugating targeting peptides (red) or fusogenic peptides to promote endosomal escape (blue) may be added. These modifications optimize protocell retention, increase drug concentrations at the tumor site and allow protocells to target different tumors [38].

Fig. 3

Covalent and non-covalent conjugation strategies for nanocarriers. (A) Schematic representation of conjugating targeting moieties to resident functional groups (red or grey spheres) on the phosopholipid head groups of the protocell outer lipid leaflet using a one- or two-step process. (B) Common single-step conjugation strategies including covalent traditional conjugation strategies, click chemistry, NTA/Ni²⁺-His₆ or hydrophobic coiled/coil interactions. (C) Linking targeting moieties that contain sulhydryl groups utilize two-step reactions that require homo- or heterobifunctional crosslinkers or clickable linkers.

Fig. 4

Schematic design of a functionalized protocell. Tumor-targeting peptide ligands or recombinant human scFvs can be conjugated directly or indirectly to functional groups on the outer leaflet of the protocell lipid bilayer. Functionalized protocells can be loaded with a wide variety of cargos such as chemotoxins, genes, siRNA, aptamers or imaging agents. The composition of the lipid bilayer can be modified to regulate the concentration of bound peptide ligands or scFvs to minimize binding site inhibition and optimize therapeutic indices, and may also incorporate different polymer coatings (purple dots) to improve circulation retention times.