Cyclodextrin-containing polymers: versatile platforms of drug delivery materials

Abstract

Nanoparticles are being widely explored as potential therapeutics for numerous applications in medicine and have been shown to significantly improve the circulation, biodistribution, efficacy, and safety profiles of multiple classes of drugs. One leading class of nanoparticles involves the use of linear, cyclodextrin-containing polymers (CDPs). As is discussed in this paper, CDPs can incorporate therapeutic payloads into nanoparticles via covalent attachment of prodrug/drug molecules to the polymer (the basis of the Cyclosert platform) or by noncovalent inclusion of cationic CDPs to anionic, nucleic acid payloads (the basis of the RONDEL platform). For each of these two approaches, we review the relevant molecular architecture and its rationale, discuss the physicochemical and biological properties of these nanoparticles, and detail the progress of leading drug candidates for each that have achieved clinical evaluation. Finally, we look ahead to potential future directions of investigation and product candidates based upon this technology.

Figure 1

Structure of linear, cyclodextrin-based polymer (CDP) for small molecule delivery. The polymer consists of the cyclical sugar β-cyclodextrin that has been difunctionalized with the natural amino acid cysteine (CDDCys) and polyethylene glycol (PEG). Two small-molecule drugs per polymer repeat unit can be attached via various linker chemistries, resulting in a neutrally charged, highly water-soluble polymer conjugate.

Figure 2

Transmission electron micrograph (TEM) of CRLX101 (from [8]).

Figure 3

Timeline of the development of cyclodextrin-containing polymers (CDPs) for nucleic acid delivery.

Figure 4

Polymerization scheme to yield amine-terminated CDP (from [32]).

Figure 5

Polymer modification scheme to incorporate imidazole derivative within CDP.

Figure 6

Effect of imidazole incorporation within CDP upon gene delivery efficiency and intracellular siRNA release. (a) Incorporation of an imidazole derivative within CDP (CDPim) leads to a significant increase in transgene (luciferase) expression levels in transfected HeLa and BHK-21 cells (from [33]). (b) Imidazole incorporation (CDPimid) yields a significant (~4x) increase in the fraction of intracellular siRNA that is released from the polymer and able to migrate through an agarose gel when electrophoresed (from [34]).

Figure 7

Formation of inclusion complexes between adamantane (AD) and β-cyclodextrin allows straightforward, noncovalent incorporation of stabilizing (via PEG-AD conjugates) and/or targeting (via ligand-PEG-AD conjugates) components to a polymer-nucleic acid nanoparticles (polyplex) (Figure from [21]).

Figure 8

Effect of AD-PEG-Tf incorporation on nanoparticle size, salt stability, and transgene efficiency. (a) Dynamic light scattering (DLS) measurements of nanoparticle size as a function of time after the addition of salt (phosphate-buffered saline) help to define an optimal formulation window above which excessive AD-PEG-Tf leads to salt-induced nanoparticles aggregation. Nanoparticles were prepared containing 0% (Ad-PEG) or the indicated mol% of AD-PEG-Tf (percentage of total cyclodextrins by mole, with the remaining balance to 100% comprised of AD-PEG). (b) When plasmid-containing nanoparticles are exposed to cultured cells, inclusion of AD-PEG-Tf in the formulation increases transgene expression in a manner that can be reversed by addition of soluble Tf as a competitor, suggesting that TfR-mediated endocytosis plays a role in nanoparticle uptake and/or intracellular trafficking. Treatments included cell alone (cells), non-AD-PEG-Tf-containing nanoparticles (PEG-part.), non-AD-PEG-Tf-containing nanoparticles plus 0.05 mol% free soluble Tf (PEG-part. + Tf), AD-PEG-Tf-containing nanoparticles (Tf-PEG-part.), non-AD-PEG-Tf-containing nanoparticles plus 10 equivalents of free soluble Tf (PEG-part. + Tf + 10x  Tf), or AD-PEG-Tf-containing nanoparticles plus 10 equivalents of free soluble Tf (Tf-PEG-part. + 10x  Tf) (from [22]).

Figure 9

RONDEL-based nanoparticles containing siRNA against EWS/Fli-1 were well tolerated by mice and efficacious in a disseminated murine model of Ewing's sarcoma. (a) When administered twice weekly for four weeks, only nanoparticles containing AD-PEG-Tf and anti-EWS/Fli-1 siRNA (D) were effective in reducing tumor burden, as measured as integrated bioluminescence. Treatment group definitions: (A): vehicle control (D5W, 5 wt% dextrose in water), (B): anti-EWS-Fli1 siRNA without any other nanoparticle components (Naked siEFBP2), (C): nontargeting siRNA within Tf-targeted nanoparticles (Targ. form siCON#1), (D): anti-EWS-Fli1 siRNA within Tf-targeted nanoparticles (Targ. form siEFBP2), (E): anti-EWS-Fli1 siRNA within non-Tf-targeted nanoparticles (Nontarg. form siEFBP2). (b) When administered once to immunocompetent mice, these nanoparticles were well tolerated with respect to liver enzymes (aspartate transaminase (AST), alanine transaminase (ALT), alkaline phosphatase (ALKP), platelets (PLTs), indicators of kidney function (blood urea nitrogen (BUN) and creatinine (CRE)), and cytokine response (IL-12(p40) and IFN-α). Wild type denotes untreated mice; all other results are indicated by treatment group letter (A–E), as defined above, and the time point (2 or 24, in hours) at which blood was sampled (from [23]).

Figure 10

Interim data from a first-in-man, phase I clinical evaluation of CALAA-01 reveals RRM2 down regulation via an RNAi mechanism of action. (a) Measurements of RRM2 mRNA or protein levels in tumor biopsies from three patients (A, B, and C2) obtained before or after CALAA-01 treatment reveal significant reductions in target expression levels. (b) 5′-RLM-RACE analysis of RNA from one patient (C2) reveals evidence of the precise RRM2 mRNA cleavage product expected from RNAi-mediated down-regulation from the C05C siRNA contained within CALAA-01. This was the first such evidence of the RNAi mechanism of action in humans of any kind (from [26]).