c‑RGDKYQ: Tyrosinase‐Responsive Melanoma Peptide

• The paper demonstrates that c‑RGDKYQ, a cyclic RGD peptide, undergoes tyrosinase-triggered oxidation to form melanin-like nanostructures for targeted melanoma therapy.
• Experimental data reveal that intracellular self-assembly disrupts actin dynamics, impairing cell adhesion and motility leading to apoptosis in melanoma cells.
• Preclinical murine studies confirmed significant tumor suppression and dose-dependent cytotoxicity with minimal systemic toxicity.

Searching arXiv for the cited paper and related background. {"query":"(Zhao et al., 9 Jul 2025)"} c‑RGDKYQ is a rationally designed, tyrosinase‑responsive cyclic peptide developed as a melanoma‑directed therapeutic that couples enzyme recognition, intracellular self‑assembly, and cytoskeletal disruption in a single molecular system. Derived from the sequence RGDKYQ and cyclized through an ortho‑phthalaldehyde (OPA)–mediated condensation, it exploits the overexpression of tyrosinase in melanoma cells to trigger oxidation of its Tyr residue, formation of melanin‑like nanostructures, and selective disruption of actin dynamics. The reported consequence is impairment of motility, adhesion, and proliferation, followed by apoptosis, with strong tumor suppression in a murine model and minimal systemic toxicity (Zhao et al., 9 Jul 2025).

1. Molecular definition and design logic

c‑RGDKYQ is based on the hexapeptide sequence RGDKYQ, comprising Arg–Gly–Asp–Lys–Tyr–Gln. Its defining structural modification is cyclization of the linear precursor under mild aqueous conditions using OPA in phosphate buffer/EtOH at approximately pH 8. OPA reacts with nucleophilic amines to generate a cyclic isoindole‑type linkage that constrains the peptide into a macrocycle. The exact covalent pattern is not spelled out in the text, but the scheme and methods indicate that the unprotected peptide contains two primary amines, the N‑terminus and the Lys side chain, and that OPA condenses with two amines to form a phthalimidine/isoindole bridge. This suggests a macrocyclic architecture in which the N‑terminal amine and Lys side‑chain amine are tied through an OPA‑derived aromatic unit (Zhao et al., 9 Jul 2025).

The sequence integrates several functional motifs. The RGD segment is an integrin‑binding motif associated with recognition of integrins such as $\alpha_v\beta_3$ and $\alpha_5\beta_1$. Lys provides a primary amine for OPA‑mediated cyclization and contributes cationic character. Tyr is the tyrosinase substrate and functions as the enzymatic switch. Gln contributes polarity and helps tune solubility and assembly behavior. The resulting construct is described as a compact enzyme‑responsive “single‑component therapeutic,” because it does not rely on an external cytotoxic payload or on a separate carrier system.

A concise description of the design elements is useful for situating the molecule:

Motif	Role in c‑RGDKYQ
RGD	Integrin‑binding and cell recognition
Lys	Cyclization handle and cationic contribution
Tyr	Tyrosinase substrate and oxidation switch
Gln	Solubility and assembly tuning
OPA‑derived bridge	Rigidity, hydrophobicity, and self‑assembly propensity

This architecture places molecular targeting, enzyme responsiveness, and supramolecular transformation within the same peptide scaffold. A plausible implication is that the therapeutic effect depends not on one isolated affinity interaction, but on coordinated progression from cell association to intracellular chemical conversion and then to higher‑order assembly.

2. Tyrosinase activation and supramolecular transformation

The mechanistic trigger for c‑RGDKYQ is tyrosinase, the melanocyte- and melanoma‑associated enzyme that participates in melanin biosynthesis. The background catalytic sequence described for tyrosinase is:

$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$

$$\text{L‑DOPA} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{Dopaquinone}$$

Within c‑RGDKYQ, the Tyr residue serves as the substrate. In melanoma cells, tyrosinase oxidizes Tyr to an $o$‑quinone species:

$$\text{Tyr–OH} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{Tyr–O–quinone}$$

The resulting quinone is described as capable of further oxidation and polymerization, intermolecular crosslinking through reactions such as Michael addition and Schiff base formation, and generation of melanin‑like aromatic oligomers with $\pi$‑stacking propensity. Because melanoma cells express high tyrosinase activity, this transformation is strongly favored in that cellular context (Zhao et al., 9 Jul 2025).

Several experimental readouts support this oxidation‑assembly process. A color progression was reported from transparent for the linear peptide to light yellow after OPA cyclization and then dark yellow after tyrosinase treatment, consistent with melanin‑like oxidation products. UV–vis spectroscopy showed new absorption peaks in the 330–450 nm range after tyrosinase treatment, and fluorescence spectroscopy with excitation at 340 nm produced strong emission at 450 nm, indicating formation of conjugated aromatic structures. FTIR analysis further supported cyclization and oxidation: OPA cyclization altered the benzene ring C–H out‑of‑plane bending near 925 cm$^{-1}$, while tyrosinase treatment reduced peaks at 1138–1208 cm$^{-1}$, associated with phenolic C–O stretching, and increased signal around 1400 cm$^{-1}$, consistent with oxidized aromatic or quinone‑like features. HPLC and mass spectrometry showed enzyme‑specific transformation, with no oxidation or assembly in the absence of tyrosinase.

Dynamic light scattering and zeta‑potential measurements indicate a shift from small, broadly distributed aggregates before tyrosinase treatment to larger, more uniform nanostructures afterward, with sizes in the tens to low hundreds of nanometers according to the figure interpretation. The reported change in zeta potential is consistent with increased hydrophobicity and colloidal stability. No explicit TEM or SEM description is given in the main text, but the interpretive model is that oxidized c‑RGDKYQ forms melanin‑like polymeric nano‑assemblies stabilized by aromatic stacking, hydrogen bonding, and quinone‑mediated covalent crosslinking.

3. Actin cytoskeleton disruption and loss of melanoma cell function

Once assembled intracellularly, c‑RGDKYQ is reported to interfere with actin dynamics. Phalloidin staining of untreated B16 melanoma cells showed dense, continuous, fibrous actin networks together with a well‑spread morphology and normal stress fibers. After 24 h of c‑RGDKYQ treatment, phalloidin fluorescence intensity was markedly reduced, actin filaments appeared fragmented, disordered, and discontinuous, and cells exhibited impaired spreading and adhesion with rounded or shrunken morphology. The interpretation given is that oxidation and assembly of c‑RGDKYQ in melanoma cells inhibit actin polymerization and compromise the growth and maintenance of actin filaments (Zhao et al., 9 Jul 2025).

Functional assays are consistent with this cytoskeletal phenotype. In wound‑healing experiments, B16 monolayers treated with c‑RGDKYQ at 0.1 mg/mL showed profoundly delayed wound closure over 24–48 h relative to control medium and relative to the linear peptide RGDKYQ. The effect was statistically significant at later time points, with $\alpha_5\beta_1$0 versus control. The corresponding qualitative interpretation is loss of lamellipodia and filopodia, impaired cell–matrix adhesion due to disturbed integrin–actin coupling, and reduced spreading with progressive detachment and rounding.

The emphasis on actin rather than microtubules is central to the system’s identity. The peptide is not described as a classical small‑molecule inhibitor of polymerization; instead, the proposed mechanism is a mechanically and biochemically disruptive supramolecular one, arising after tyrosinase‑mediated transformation of the peptide precursor into nanostructures. This suggests that cytoskeletal collapse is an emergent consequence of localized intracellular assembly rather than of stoichiometric occupation of a canonical actin binding pocket.

4. Melanoma selectivity and apoptosis induction

Selectivity is attributed primarily to tyrosinase overexpression in melanoma cells. B16 melanoma cells are described as having significantly higher tyrosinase activity than L929 fibroblasts and U2OS osteosarcoma cells, enabling efficient conversion of c‑RGDKYQ into its oxidized, assembling form. In vitro viability data from CCK‑8 assays showed that linear RGDKYQ had negligible cytotoxicity up to the tested concentrations in B16, L929, and U2OS cells, whereas cyclic c‑RGDKYQ caused concentration‑dependent cytotoxicity in B16 cells with minimal effects in L929 and U2OS. The trend is especially evident at 100–200 $\alpha_5\beta_1$1g/mL, although exact IC$\alpha_5\beta_1$2 values are not printed in the text (Zhao et al., 9 Jul 2025).

Apoptosis was measured by Annexin V‑FITC/PI flow cytometry after 24 h exposure to 0, 50, 100, and 200 $\alpha_5\beta_1$3g/mL c‑RGDKYQ. The apoptotic fraction increased progressively with dose, and the highest dose produced marked apoptosis with a visually substantial total apoptotic population. One‑way ANOVA showed a significant dose‑dependent increase, including ***$\alpha_5\beta_1$4 at high doses. Live/dead staining with Calcein AM/PI also showed a dose‑dependent loss of live cells and increase in dead cells under confocal microscopy.

The mechanistic chain presented for apoptosis is:

$\alpha_5\beta_1$5

The study further relates this chain to known cytoskeletal survival pathways, noting that disrupted actin architecture can impair focal adhesion signaling through FAK/Src, compromise AKT and ERK survival pathways, induce anoikis, and perturb cytoskeletal tension and mitochondrial positioning. At the same time, the report explicitly notes the absence of direct Western blot data for caspase‑3 cleavage, Bcl‑2/Bax modulation, or other pathway markers. Accordingly, the apoptosis mechanism is supported by phenotype, histology, and flow cytometry rather than by a fully dissected signaling analysis.

The RGD motif may contribute an additional level of selectivity through association with integrins upregulated in tumor environments. The discussion states that cyclic RGD peptides typically show enhanced integrin affinity relative to linear RGD and may localize the peptide near integrin clusters and focal adhesions. However, no integrin‑blocking or receptor‑binding experiments are reported, so the integrin‑targeting contribution remains a design rationale rather than a directly isolated causal factor in this study.

5. Preclinical efficacy and systemic safety

In vivo evaluation used a subcutaneous B16 melanoma model in 7‑week‑old female nude mice. Tumors were established by injection of $\alpha_5\beta_1$6 B16 cells in 200 $\alpha_5\beta_1$7L into the right flank or dorsal side. Tumor volume was calculated as

$\alpha_5\beta_1$8

where $\alpha_5\beta_1$9 is the longest diameter and $$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$0 the shortest. Treatment began after tumors reached at least 30 mm$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$1, approximately 7–10 days after grafting (Zhao et al., 9 Jul 2025).

The therapeutic regimen involved peri‑tumoral injection of c‑RGDKYQ or linear RGDKYQ in sterile saline at 7.5 mg/kg, with repeated dosing during early tumor growth. Two related descriptions are reported: one centered on injections on day 3 and day 6 in the figure, and one in the methods section describing injections every 3 days for a total of four injections. Both accounts convey local repeated administration after tumor establishment.

Antitumor efficacy was substantial. Tumor growth curves showed marked suppression of tumor volume in the c‑RGDKYQ group relative to saline and relative to linear RGDKYQ, with statistical significance increasing over time from $$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$2 to **$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$3. Final tumor weights were likewise significantly lower in the c‑RGDKYQ group. Macroscopic images showed visibly smaller tumors after c‑RGDKYQ treatment, whereas saline and RGDKYQ groups exhibited large bulky tumors. Histological examination with H&E showed that control and linear RGDKYQ tumors retained dense viable tumor cell populations, while c‑RGDKYQ tumors contained increased apoptotic or necrotic regions, reduced cellular density, and morphologic hallmarks of cell death.

Systemic toxicity was reported as minimal. Body‑weight curves showed no significant changes among control, RGDKYQ, and c‑RGDKYQ groups throughout the study. Organ histopathology included lung and tumor tissues, and the main text states that no major systemic toxicity was evident and that no obvious damage in major organs was reported. The conclusion of minimal systemic toxicity is therefore based on stable body weight, absence of overt illness or mortality beyond tumor burden, and lack of reported organ pathology.

6. Relation to cyclic RGD literature, therapeutic positioning, and generalization

c‑RGDKYQ belongs to a broader class of cyclic RGD‑containing systems in which conformational constraint is used to preserve receptor‑compatible geometry while tuning presentation and function. A molecular dynamics study of c(RGDyK) on PEGylated TiO$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$4 nanoparticles showed that cyclic RGD ligands can retain a backbone geometry close to the integrin‑bound state and that ligand density strongly regulates exposure: low to moderate densities of 0.2–0.5 ligands nm$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$5 favored a straight upward orientation and high water accessibility, whereas 1.1 ligands nm$$\text{L‑Tyrosine} \xrightarrow[\text{O}_2]{\text{Tyrosinase}} \text{L‑DOPA}$$6 promoted clustering, reduced hydration, and partial burial within the PEG layer (Siani et al., 2022). Although that work addressed PEGylated nanoparticles rather than c‑RGDKYQ itself, it is relevant because the c‑RGDKYQ study explicitly situates its RGD element within cyclic RGD design logic and notes that cyclic RGD peptides are generally associated with enhanced receptor selectivity and binding.

Against conventional melanoma therapy, c‑RGDKYQ is distinguished in the primary report by four design features: it carries no exogenous drug payload; it does not require liposomes, polymeric nanoparticles, or antibodies; it is activated in situ by an endogenous enzyme; and it aims to localize cytoskeletal disruption through melanoma‑specific tyrosinase rather than through systemic exposure to generic cytoskeletal poisons (Zhao et al., 9 Jul 2025). The study therefore presents the peptide as an enzyme‑responsive alternative to chemotherapy, targeted therapies limited by rapid resistance, and immunotherapies that have incomplete response rates and potentially severe immune‑related adverse events. A plausible implication is that the mechanism may be less vulnerable to classical single‑target mutational resistance, because the proximate insult is physical interference with cytoskeletal organization after endogenous enzymatic activation.

The generalization proposed in the report follows a design blueprint: identify a tumor‑specific enzyme, incorporate its substrate amino acid or motif into a peptide, embed self‑assembly‑promoting elements such as aromatics or cyclization, optionally add a targeting motif such as RGD, and engineer the sequence so that enzyme action converts a soluble precursor into an assembling species that exerts a mechanical or biophysical therapeutic effect. In the specific case of c‑RGDKYQ, the sequence comparison between linear RGDKYQ and cyclic c‑RGDKYQ indicates that cyclization and the resulting oxidation‑assembly behavior are crucial for activity, because the linear peptide was non‑toxic, caused no major cytoskeletal disruption, and showed poor efficacy in vivo.

Several limitations remain explicit. The report does not include direct actin‑binding or in vitro polymerization assays, detailed apoptotic signaling blots, pharmacokinetic or biodistribution measurements, or integrin‑blocking experiments. The in vivo evaluation is restricted to a single B16 melanoma model in nude mice and uses local peri‑tumoral administration rather than systemic delivery. These constraints do not negate the reported findings, but they define the current evidentiary boundary: c‑RGDKYQ is best understood as a preclinical demonstration of tyrosinase‑triggered peptide self‑assembly for selective melanoma apoptosis, with molecular and translational questions still open for further study.