Temperature sensitive peptides: Engineering hyperthermia-directed therapeutics

Figures & data

Table I.  Categories of thermo-responsive peptides.

Category	Response to heating	Reversible	Example	Peptide motif *	Trends**	Secondary structure	Higher order behavior	Refs
I	Disassociates	Yes	Leucine zipper	[abcdefg]n	a to g ≠ P a = hydrophobic d = L e, g = charged n = 5 to 6	α helix	Coiled coil	[11], [12], [18], [19], [21], [24]
II	Disassociates	No	Collagen	[Gab]n	a = P b = P^OH n = 100 to 500	triple helix	Fibrils	[11], [12], [24–26]
III	Associates	Yes	Elastin	[aPGbG]n	a = I or V b ≠ P n = 10 to 200	β turn spirals	Phase separates	[11], [12], [24], [27], [31]
IV	Associates	No	Silk	[GAGAGS]n	n = 2 to 168	β sheets	Crystalline aggregates	[11], [12], [24], [31], [32]

*Variable amino acids are indicated in lowercase and italicized for clarity. Uppercase amino acids indicate a nearly conserved amino acid using single letter code. A = alanine, G = glycine, I = isoleucine, L = leucine, P = proline, P^OH = hydroxyproline, S = serine, V = valine. **General trends observed in literature are indicated.

Figure 1. Strategies for designing anti-tumor temperature sensitive peptides. Images depict peptide drug carriers flowing through permeable tumor vasculature during or after local hyperthermia. (A) An associating peptide accumulates into depots within hyperthermic tumor regions. (B) A disassociating peptide micelle disassembles within hyperthermic tumor, and monomeric peptides diffuse rapidly into the tumor. (C) An irreversible-associating peptide is retained in depots after the tissue cools to body temperature, contacting a subset of cells. (D) A reversible-associating peptide floods the tissue with concentrated monomeric peptides after the tissue cools to body temperature, contacting most of the cells.

Figure 2. Elastin-like polypeptide block copolymers: affinity modulated switches. (A) An ELP block copolymer with a hydrophilic [V₁A₈G₇-96] amino terminus and a hydrophobic [V₅-60] carboxy terminus. Also indicated are an amino terminal NGR peptide ligand (half-ring) and a carboxy terminal therapeutic agent (lightning bolt). At body temperature the ELPs are monomeric, but at 42°C the ELPs assemble into spherical micelles decorated with multiple ligands. (B) Hydrodynamic radius and optical density as a function of temperature for ELP block copolymers with and without the NGR peptide ligand. For 25 µM peptide, Both ELPs are monomeric at 37°C and multimeric at 42°C. Error bars indicate polydispersity. (Reproduced with permission [30]).

Figure 3. Pharmacokinetics and tumor accumulation of elastin-like polypeptides. ¹⁴C labeled ELPs were administered via tail vein to nude mice for an initial plasma concentration of 25 µM. (A) Whole blood pharmacokinetics were determined over 24 hours. The ELP1[V₅A₂G₃-150] concentration was determined and fit with a bi-exponential solution to a two compartment model to obtain the indicated half-lives. (B) A temperature sensitive ELP1[V₅A₂G₃-150] (T_t = 39°C) and a temperature insensitive ELP2[V₁A₈G₇-160] (T_t = 53°C) were administered to mice with subcutaneous FADU tumors in the right hind leg. Immersed in a water bath, the leg and tumor were heated to 41.5°C for 1 h before isolation. * indicates that ELP1(heat) was significantly (p < 0.05, Fischer's PLSD) greater than either control. Error bars indicate the standard error of the mean (n = 5). (Reproduced with permission [33]).

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