Flavoplatins: photoactivated platinum(IV) prodrugs bearing axial N-donors that trigger pyroptosis and reduce drug resistance

Abstract

Photoactivated Pt(IV) anticancer prodrugs derived from clinical Pt(II) drugs have garnered significant attention in recent years, with O-donor ligands being the most commonly used photosensitive axial ligands. N-donor ligands, however, such as N-heteroaromatics, offer the potential to enhance the ligand-to-metal charge transfer (LMCT) of Pt(IV) complexes, thereby improving their photochemical properties. Herein, we report a series of green-light activable Pt(IV) prodrugs bearing N-donor axial ligands, designated flavoplatins, based on carboplatin and oxaliplatin. These prodrugs, functionalized with flavonol derivatives as photosensitive axial ligands, enabled rapid reduction under light irradiation to release Pt(II) drugs and the corresponding axial ligands. Flavoplatins 3a and 3b demonstrated exceptional photocytotoxicity, exhibiting at least a 27-fold increase in effectiveness compared to carboplatin in both Pt-sensitive and Pt-resistant cancer cells. Additionally, both complexes efficiently accumulated in the endoplasmic reticulum and quickly induced pyroptosis via the NLRP-3/caspase-1/GSDMD pathway. This study underscores a promising alternative strategy for designing novel photoactivatable Pt(IV) prodrugs containing axial N-donors with enhanced therapeutic potential, particularly for targeting specific cellular pathways and reducing drug resistance.

---

Introduction

Flavonoids are an important class of natural products found in fungi and plants,^1,2 featuring a heterobicyclic structure with a phenyl group substituted at the 2-position.³ Based on structural variations, flavonoids are categorized into several subgroups, including flavonols,⁴ which are extensively studied for their anticancer properties.^5,6 Among the notable flavonols, quercetin–a dietary compound present in many fruits and vegetables–has shown broad-spectrum anticancer activity and progressed to phase II clinical trials.^7,8 Other flavonols have been reported to actively disrupt cell cycles, regulate signaling enzymes, and induce apoptosis in cancer cells.^9–11 Flavonols also exhibit endoplasmic reticulum (ER)-targeting properties, largely due to their strong hydrophobic interactions with the ER.^12,13 Further modifications to flavonols, such as the addition of sulfonated substituent groups, have been shown to enhance their ER-targeting ability.¹⁴ Additionally, the excited-state intramolecular proton-transfer (ESIPT) properties of flavonols result in strong and long-wavelength emissions in their excited tautomeric forms,¹⁵ enabling their application in whole-cell imaging and organelle probing.^12,16

Since the discovery of cisplatin as the first anticancer platinum drug, significant efforts have been devoted to developing novel Pt-based anticancer agents.¹⁷ For example, Pt(IV) prodrugs derived from Pt(II) drugs including cisplatin, carboplatin, and oxaliplatin have been widely reported.^18–21 A critical aspect of designing these prodrugs is their reduction to release the active Pt(II) species, which drives their anticancer activity.^22–24 O-donor ligands are commonly employed as axial ligands to confer multifunctionality on Pt(IV) anticancer prodrugs based on clinical Pt(II) drugs.^21,25,26 In contrast, N-donor ligands are less commonly reported. Notable examples include amidates and pyridines, which can also coordinate axially to the Pt(IV) center through either oxidation of Pt(II) drugs or ligand exchange reactions involving labile axial ligands.^27–29 It is noteworthy that axial amidates and pyridine ligands can be efficiently released during the reduction of Pt(IV) complexes.^27–29 This observation highlights axial N-coordination as a promising strategy for designing Pt(IV) anticancer prodrugs. Furthermore, these synthetic approaches allow diverse modifications of the axial ligands, enabling the fine-tuning of reduction profiles, conjugation with bioactive molecules,²⁹ or incorporation of photosensitive ligands.

Photoactivation has recently emerged as a controllable method for activating Pt(IV) prodrugs to enhance their anticancer potency and selectivity.^30–37 These prodrugs exhibit significant cytotoxicity and additional bioactivities upon irradiation, such as generating reactive oxygen species (ROS) and inducing photo-oxidation of cellular components.^37–39 Currently, most photosensitive axial ligands in clinical Pt(II) drug-based Pt(IV) complexes are conjugated via O-coordination, while N-coordinated axial ligands are rarely reported. Intriguingly, N-donor ligands exhibit ligand-to-metal charge transfer (LMCT) properties, which narrow the energy gap between the ground state and the excited state, destabilizing the Pt–N bond upon excitation.⁴⁰ Consistent with this behavior, azido, pyridine, or ethylenediamine N-donors enable LMCT-mediated activation of Pt(IV) complexes under short-wavelength visible light.^31,32,41 Among these, pyridine–as an N-heteroaromatic scaffold–offers synthetic versatility for conjugation with photosensitive molecules. More broadly, N-heteroaromatic ligands exhibit strong electron transfer abilities, enhance photosensitivity, and are widely utilized in photocatalytic metal complexes.⁴² As a result, the LMCT bands of Pt(IV) complexes containing N-heteroaromatic ligands are significantly red-shifted,⁴³ enabling activation by longer wavelengths of light. This property enhances the photosensitivity of Pt(IV) prodrugs and facilitates the adjustment of activation wavelength.

Herein, we report the design and synthesis of a series of green-light-activable Pt(IV) prodrugs featuring flavonol-derived N-donor axial ligands. Photocytotoxicity assays demonstrated a significant increase in activity under irradiation compared to dark conditions, highlighting their light-triggered therapeutic potential. Notably, flavoplatins preferentially localized to the ER and induced pyroptosis through well-characterized pathways, which contributed to reducing platinum drug resistance. This study introduces a novel approach to designing photoactivatable Pt(IV) prodrugs bearing axial N-donors and highlights the potential of such agents to achieve enhanced anticancer potency and selectivity through light-controlled activation.

Results and discussion

Previously, we reported a synthetic method for coordinating N-heteroaromatic ligands to Pt(IV) complexes utilizing a ligand exchange reaction with the axial halogen ligands of Pt(IV) complexes.²⁹ Building on this approach, flavonol analogs bearing a pyridinyl moiety were rationally designed as axial ligands for Pt(IV) complexes and synthesized following the previously reported method.²⁹ In brief, 3-hydroxy-4-acetylpyridine underwent a condensation reaction with 4-diethylaminobenzaldehyde under basic conditions, yielding a chalcone-derived intermediate. After neutralization with nitric acid, hydrogen peroxide solution was directly added to the reaction mixture, oxidizing the chalcone derivative to produce the ligand flav-1 (Scheme S1A). Ligands flav-2 and flav-3 were subsequently obtained by reacting flav-1 with acetic anhydride and benzenesulfonyl chloride, respectively (Schemes S1B and S1C). All synthesized ligands were characterized using ¹H nuclear magnetic resonance (NMR; Fig. S1 to S3) spectroscopy and high-resolution mass spectrometry (HRMS).

By following our previously reported method,²⁹ the brominated Pt(IV) complex cis,trans-[Pt(NH₃)₂(CBDCA)(OH)Br] was reacted with silver nitrate to yield the aquated intermediate cis,trans-[Pt(NH₃)₂(CBDCA)(OH)(OH₂)](NO₃), where CBDCA is 1,1-cyclobutanedicarboxylate. The silver bromide precipitate was filtered off before the addition of flav-1, resulting in the first desired Pt(IV) complex cis,trans-[Pt(NH₃)₂(CBDCA)(OH)(flav-1)](NO₃), designated flavoplatin 1a (Fig. 1 and Scheme S2A). Similarly, starting from cis,trans-[Pt(NH₃)₂(CBDCA)(OH)Br], flavoplatins 2a and 3a were respectively synthesized through ligand exchange reactions with flav-2 and flav-3 (Scheme S2A). Additionally, we explored the modification of the axial hydroxido ligand of flavoplatins. Reactions of flavoplatins 2a and 3a with acetic anhydride converted them into cis,trans-[Pt(NH₃)₂(CBDCA)(OCOCH₃)(flav-2)](NO₃) (2b) and cis,trans-[Pt(NH₃)₂(CBDCA)(OCOCH₃)(flav-3)](NO₃) (3b), respectively, while the carboxylated product of flavoplatin 1a was not obtained under the same condition (Fig. 1 and Scheme S2A). For the oxaliplatin-based brominated Pt(IV) complex, the ligand exchange reaction was modified by using an acetonitrile/dimethyl sulfoxide (DMSO; 3 : 1) solvent mixture. This procedure yielded another flavoplatin trans-[Pt(DACH)(ox)(OH)(flav-3)](NO₃) (3c), synthesized by coordinating flav-3 to the oxaliplatin-based Pt(IV) complex (Scheme S2B). The flavoplatins were characterized by ¹H, ¹³C, ¹⁹⁵Pt NMR spectroscopy and HRMS (Fig. S4–S18). Their purities were confirmed by high-performance liquid chromatography (HPLC; Fig. S19).

Fig. 1 Chemical structures of flavoplatins 1a, 2a, 2b, 3a, 3b, and 3c.

The hydrolytic stability of flavoplatins in the dark was determined using HPLC (Fig. S20–S25). All the tested complexes displayed a half-life of over 24 h in HEPES buffer. Among them, flavoplatins 3b and 3c were the most stable, remaining 95% and 98% intact after 24 h of incubation, respectively (Fig. 2A). Flavoplatin 2b also displayed moderately high stability, with 79% of the complex intact after 24 h. This suggests that the introduction of an axial acetato ligand may contribute to the stabilization of these complexes against hydrolysis. A comparison of Pt(IV) complexes bearing different flavonol ligands revealed an additional trend: flav-3-containing complexes were more stable than those bearing flav-1 or flav-2 ligands (3b > 2b; 3a > 1a > 2a). The hydrolysis products of the flavoplatins were analyzed by liquid chromatography-mass spectrometry (LC-MS). During hydrolysis, the flav-1 and flav-3 ligands were released from flavoplatins 1a and 3a, respectively (Fig. S20 and S23). In contrast, the N–Pt bond in flavoplatin 2a remained stable, with hydrolysis leading instead to cleavage of the acetate group from the flav-2 ligand (Fig. S21).

Fig. 2 (A) and (B) Stability tests of flavoplatins 1a, 2a, 2b, 3a, 3b, and 3c; 200 μM flavoplatin solutions were prepared in 50 mM HEPES buffer (20% DMF, pH 7.4) and incubated at 37 °C. Solutions were monitored by HPLC from 0 to 24 h. (B) Flavoplatin solutions with 2 mM sodium ascorbate (pH 7.4) were monitored by HPLC from 0 to 24 h. (C) HOMO and LUMO of flav-3 and flavoplatin 3a. (D) Absorption spectrum of 25 μM flav-3 and emission spectrum of 10 μM flav-3 excited at 405 nm, in 50 mM HEPES buffer (20% DMF, pH 7.4). (E) Absorption spectrum of 25 μM flavoplatin 3a and emission spectrum of 10 μM flavoplatin 3a excited at 488 nm; in 50 mM HEPES buffer (20% DMF, pH 7.4). (F) Photoactivation tests of flavoplatins 1a, 2a, 2b, 3a, 3b, and 3c; 200 μM flavoplatin solutions were prepared in 50 mM HEPES buffer with 2 mM sodium ascorbate (20% DMF, pH 7.4), irradiated by a green LED (495 nm, 0.9 mW cm⁻²) for 0 to 60 min at 37 °C, and immediately monitored by HPLC.

We conducted additional tests to assess the stability of flavoplatins in the presence of sodium ascorbate (Fig. 2B and S26–S31). Flavoplatin 3c exhibited the highest stability, with over 80% remaining after 24 h, followed by flavoplatins 2b (51%) and 1a (44%, Table S1). The other flavoplatins displayed moderate stability in the presence of the reducing agent, with a half-life of approximately 4 h (2a, 3.7 h; 3a, 4.2 h; 3b, 4.4 h; Fig. 2B and Table S1). The reduction products of all tested complexes were analyzed using HRMS. Flavoplatins are predominantly reduced to carboplatin or oxaliplatin, releasing the corresponding axial flavonol ligands (Fig. S32). During reduction, the hydrolysis of the acetate group in flavoplatin 2a occurred, resulting in the minor product ligand flav-1 (Fig. S27). Intriguingly, reductive ligand rearrangements were observed in flavoplatins 1a, 3a, and 3b; in these cases, the flavonol ligands remained intact; through the loss of an amine and the axial hydroxido or acetato ligand, a class of flavonol-bearing Pt(II) complexes was formed as minor products (Fig. S26, S29, and S30).

To investigate the excitation profiles of the flavonol ligands and flavoplatins, we selected flav-3 and flavoplatin 3a for computational analysis. Time-dependent density functional theory (TD-DFT) calculations showed that the lowest singlet excited state (S₁) in both species was primarily dominated by a HOMO to LUMO transition. For flav-3, both frontier orbitals reside largely on the ligand, consistent with a typical intra-ligand (IL) excitation. In contrast, for flavoplatin 3a, the HOMO was mainly distributed on the flav-3 ligand, whereas the LUMO was centered on the Pt(IV) moiety, indicating pronounced ligand-to-metal charge transfer (LMCT). This strong LMCT character reduced the S₀ to S₁ energy gap following the conjugation of flav-3 (ΔE = 2.87 eV, λ = 432 nm) to the Pt(IV) center (flavoplatin 3a: ΔE = 2.33 eV, λ = 533 nm; Table S2). This decrease resulted in absorption at longer wavelengths. Consistent with computations, flavoplatin 3a exhibited a significant red shift in the experimental absorption maximum (λ_max = 484 nm) compared with flav-3 (λ_max = 442 nm; Fig. 2D and E), supporting the presence of an LMCT band driven by aromatic N-coordination. Consequently, we employed a blue LED (λ_max = 425 nm, 1.0 mW cm⁻²) to investigate the photoactivation profiles of ligands flav-1 to flav-3, and a green LED (λ_max = 495 nm, 0.9 mW cm⁻²) for flavoplatins 1a to 3c (Fig. S33). Furthermore, we recorded the emission spectra of flav-3 and flavoplatin 3a, with the fluorescence wavelengths centered at 601 nm and 653 nm, respectively (Fig. 2D and E).

The activation of flavoplatins in HEPES buffer under green light irradiation was monitored using HPLC, and the photoreaction quantum yield was calculated and presented in Table S3. Most flavoplatins were efficiently activated within 1 h of irradiation (Fig. 2F). Flavoplatin 3c exhibited lower activation efficiency, with 52% remaining intact in the buffer after exposure. During the photoactivation process, flavoplatins were reduced to carboplatin and oxaliplatin, with the detachment of flavonol ligands flav-1, flav-2, and flav-3 (Fig. S34–S35, Scheme S3).

Next, we tested the cytotoxicity of flavoplatins and the corresponding ligands against ovarian cancer cells, as well as the mixture of Pt(II) drugs and the ligands (Fig. 3 and Table S3). As controls, cisplatin, carboplatin, and oxaliplatin showed no light-dependent change in cytotoxicity, although cisplatin and oxaliplatin displayed moderate cytotoxicity in Pt-sensitive cells (Table S4). Under blue light irradiation, ligands flav-1 and flav-2 demonstrated up to an 11.7-fold increase in cytotoxicity relative to dark conditions. A 1 : 1 mixture of flav-1 and carboplatin produced comparable photocytotoxicities in A2780 and A2780cisR cells, yielding a low resistance factor (RF) of 1.1. Similarly, the mixtures of flav-3 with carboplatin or oxaliplatin also reduced resistance upon irradiation (RFs 1.0 and 1.1). Under green-light irradiation, the Pt(IV) prodrug flavoplatin 1a exhibited IC₅₀ values of 79.3 μM and 84.3 μM against A2780 and A2780cisR cells, respectively, with a low RF of 1.1 (Fig. 3 and Table S3), whereas flavoplatins 2a and 2b were not photocytotoxic. In contrast, green-light-activated flavoplatins 3a and 3b achieved IC₅₀ values of 14.2–18.3 μM (PI > 5.4) in A2780 and A2780cisR cells, with RFs as low as 0.8, representing >35-fold greater photocytotoxicity than carboplatin and up to 20-fold greater than the flav-3/carboplatin mixture. The oxaliplatin-based flavoplatin 3c was also active under green light irradiation and reduced drug resistance. Overall, flavoplatins 1a and 3a to 3c showed enhanced cytotoxicity under green-light irradiation, whereas the free flavonol ligands are preferentially activated by blue light, aligning with their maximum absorption wavelength. Notably, the sulfonated flavonol-derived ligand flav-3 acted as a robust photosensitive ligand for Pt(IV) prodrugs, making flavoplatins 3a to 3c promising candidates for further investigation into their mechanisms of action.

Fig. 3 Photo-cytotoxicity assays in A2780 and A2780cisR cells, which were treated with the compounds for 2 h, followed by blue light (425 nm, 1.0 mW cm⁻²) or green light irradiation (495 nm, 0.9 mW cm⁻²) for 1 h. The cells were incubated for another 21 h before the MTT assays, with the total treatment time being 24 h. †IC₅₀ > 500 μM; ‡IC₅₀ > 100 μM.

We subsequently evaluated the cellular accumulation and genomic DNA binding levels of the flavoplatins. Compared to carboplatin, flavoplatin 3a exhibited a 5.6-fold higher accumulation level in A2780cisR cells. Similarly, flavoplatins 1a, 3b, and 3c displayed 2.4- to 2.7-fold increases in accumulation levels relative to their parent Pt(II) drugs (Fig. 4A). Without irradiation, the levels of Pt in the genomic DNA of cells treated with flavoplatins 1a, 3a, 3b, and 3c were comparable to those observed with Pt(II) drugs (Fig. 4B). In contrast, these levels of Pt increased by 1.8- to 2.5-fold under green light irradiation, demonstrating that these Pt(IV) prodrugs, including flavoplatin 3c, were activated efficiently in the cells. Additionally, the ability of flavoplatins to damage DNA under irradiation was confirmed by the increased expression levels of γ-H2AX in A2780cisR cells (Fig. S36).

Fig. 4 (A) Pt accumulation levels in A2780cisR cells treated with 20 μM carboplatin, oxaliplatin, and flavoplatins 1a, 3a, 3b, and 3c for 2 h. (B) Genomic DNA binding levels of A2780cisR cells treated for 2 h with 50 μM flavoplatins 1a, 3a, 3b, and 3c, followed by green light irradiation (495 nm, 0.9 mW cm⁻²) for 1 h, and the total treatment time was 24 h. The cells treated with 50 μM cisplatin, carboplatin, and oxaliplatin for 24 h in the dark were included as the positive controls.

After evaluating cellular accumulation and genomic DNA binding levels, we carried out cell cycle arrest and PI/annexin V double staining assays to determine the mode of cell death induced by flavoplatins. Cisplatin arrested the cancer cells in the S phase. In contrast, upon green light irradiation, flavoplatins 1a, 3a, 3b, and 3c arrested A2780cisR cells in the G₂/M phase (Fig. 5A and S37). Moreover, flavoplatins 3a, 3b, and 3c significantly increased the percentage of cells in late apoptosis and late necrosis, while compared to cisplatin and oxaliplatin, fewer cells remained in early apoptosis (Fig. 5B and S38). These results suggest that flavoplatins 3a, 3b, and 3c exhibit a different mechanism of action compared to the conventional platinum drugs.

Fig. 5 (A) Cell cycle arrest and (B) apoptotic cells analysis of A2780cisR cells treated for 2 h with 50 μM flavoplatins 1a, 3a, 3b, and 3c, followed by green light irradiation (495 nm, 0.9 mW cm⁻²) for 1 h, and the total treatment time was 5 h. The cells treated with 200 μM cisplatin, carboplatin, and oxaliplatin for 5 h in the dark were included as the positive controls. (C) Cell morphology of A2780cisR cells treated with 100 μM flav-1 and flav-3 for 2 h, and 50 μM flavoplatins 1a, 3a, 3b, and 3c for 2 h. The treated cells were irradiated by green light (495 nm, 0.9 mW cm⁻²) for 1 h, while the untreated group was kept in the dark. Scale bar represents 20 μm. (D) Co-localization of ER tracker green and flavoplatins in A2780cisR cells, treated with 25 μM flavoplatins 3a, 3b, and 3c, as well as 25 μM flav-3 for 2 h. Cells were imaged by a Laser Confocal Scanning Microscope; blue channel was excited at 405 nm, emission was recorded at 570 to 630 nm; green channel was excited at 488 nm, emissions were recorded at 500 to 520 nm and 625 to 675 nm. Scale bar represents 10 μm. (E) Fluorescence of a ROS probe (deep red) with the excitation at 635 nm, and emission was recorded at 660 to 680 nm. A2780cisR cells treated with 50 μM flavoplatins 3a, 3b, and 3c for 2 h. The flavoplatin-treated cells were irradiated by green light (495 nm, 0.9 mW cm⁻²) for 30 min, while the untreated cells were kept in the dark. Scale bar represents 10 μm. Western blotting images of (F) NLRP3 and pro-caspase-1, as well as (G) GSDMD and its N-terminal fragments. A2780cisR cells were treated with 50 μM flavoplatins 3a, 3b, and 3c for 2 h. The flavoplatin-treated cells were irradiated by green light (495 nm, 0.9 mW cm⁻²) for 1 h, while the untreated cells were kept in the dark. The cells treated with 1 mM MnCl₂ and 200 μM cisplatin for 4 h in the dark were included as the positive controls. The representative results were selected from three independent experiments. (H) Quantified expression levels of NLRP3; (I) pro-caspase-1; (J) GSDMD, and (K) GSDMD N-terminal fragment. *** p ≤ 0.001, ** p ≤ 0.01, * 0.01 < p ≤ 0.05, ns: not significant.

To further study the mechanisms of action, the morphologies of cells treated with flavoplatins and subjected to irradiation were imaged and analyzed. The presence of pyroptotic bodies and cell swelling in A2780cisR cells treated with flavoplatins 3a to 3c suggested that the mode of cell death was likely pyroptosis (Fig. 5C). Since the pyroptotic pathways involving ER oxidative stress have been documented,^44,45 we subsequently investigated the ER targeting ability and ROS generation of flavoplatins. Intracellular imaging was carried out in the A2780cisR cells treated with flavoplatins and flav-3, after incubation with a fluorescent ER tracker. Ligand flav-3 in the cells emitted intense fluorescence upon the excitation of blue light (405 nm; Fig. 2D and 5D), while flavoplatins 3a, 3b, and 3c were excited by green light (488 nm) and fluorescent (Fig. 2E and 5D). These observations revealed that the free ligand flav-3 and the Pt(IV) complexes could be distinguished by different excitation wavelengths of light. Notably, the sulfonated flavoplatins 3a, 3b, and 3c, as well as the free ligand flav-3, colocalized well with the ER tracker, with Pearson correlation coefficients (PCCs) ranging from 0.79 to 0.93 (Fig. 5D). In contrast, flavoplatin 1a, which was a sulfonate-free Pt(IV) complex, did not target the ER (Fig. S39). Additionally, flavoplatins 3a, 3b, and 3c upon irradiation generated ROS in the cells (Fig. 5E and S40), demonstrating these Pt(IV) prodrugs likely trigger ER oxidative stress.

Finally, we assessed the expression levels of proteins involved in ER stress pathways, including NLRP3 inflammasome,^44,45 to confirm the pyroptosis induced by flavoplatins 3a to 3c. Gasdermin D (GSDMD) and gasdermin E (GSDME) are essential proteins involved in pyroptosis, responsible for membrane pore formation following cleavage by caspases.⁴⁶ We subsequently evaluated the NLPR3/caspase-1/GSDMD pathway in A2780cisR cells treated with flavoplatins and irradiated (Fig. 5F and G), using cisplatin and a manganese salt as controls, as they have been reported to trigger GSDMD cleavage through certain mechanisms.^47,48 The results demonstrated that flavoplatins 3a 3b, and 3c effectively increased the expression of NLRP-3 (Fig. 5H), in which flavoplatins 3a and 3b further decreased the levels of downstream pro-caspase-1 by 39% and 28%, respectively (Fig. 5I). Notably, over 3-fold increased expression of the GSDMD N-terminus was observed in flavoplatin 3a-treated groups, with the levels of GSDMD decreased by 26% (Fig. 5J and K). The cleavage of GSDMD was also observed in the cisplatin- and manganese-treated groups, aligning with previous reports.^47,48 Next, we examined the caspase-3/GSDME pathway by blotting the corresponding proteins, and a doxorubicin-treated group was included as a control (Fig. S41). The results showed that only doxorubicin effectively activated caspase-3 and degraded GSDME. The N-terminal of GSDME, which is responsible for pore formation, was observed in doxorubicin-treated cells but not in flavoplatin-treated cells.⁴⁹ These findings indicate that flavoplatins 3a and 3b, upon irradiation, actively induced pyroptosis in A2780cisR cells via the NLPR3/caspase-1/GSDMD pathway. These Pt(IV) prodrugs are distinct from conventional apoptosis-inducing Pt(II) drugs and provide an approach to overcoming Pt-drug resistance.

Conclusions

In summary, we developed a series of photoactivable Pt(IV) anticancer prodrugs by conjugating flavonol-derived ligands to Pt(IV) complexes through N-coordination. This approach enhanced LMCT and enabled activation using green light. Upon light irradiation, most of these Pt(IV) complexes were quickly reduced to either carboplatin or oxaliplatin. The sulfonate-containing flavoplatins 3a, 3b, and 3c efficiently accumulated in the ER and effectively killed platinum-resistant cancer cells when irradiated, exhibiting high photo-indexes. Following the observation of pyroptotic cell morphologies and ROS generation, the NLRP3/caspase-1/GSDMD pathway of pyroptosis was confirmed. This work introduces a novel strategy for designing photoactivable Pt(IV) prodrugs, significantly improving cytotoxicity compared to non-irradiated conditions and reducing drug resistance by inducing pyroptosis. Extending the excitation wavelength through N coordination offers a route to near-infrared (NIR)-activatable Pt(IV) prodrugs.

Author contributions

Q. Z. and G. Z. designed the study. Q. Z. and N.-H. C. performed the experiments. J. X. performed the DFT calculations. Q. Z., K.-C. L. and G. Z. analyzed the data. Q. Z. and G. Z. wrote the paper. All authors edited and approved the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article (Instruments and methods, Experimental sections, Schemes S1 to S3, Fig. S1 to S43, and Tables S1 to S4) have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6qi00078a.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 22077108 and 22277103), the Hong Kong Research Grants Council (Grant No. CityU 11313222, CityU 11304923, CityU 11306724, and CityU C1018-23G), the Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20210324120004011), and the City University of Hong Kong (Grant No. 9610605) for funding support.

References

1. A. N. Panche, A. D. Diwan and S. R. Chandra, Flavonoids: an overview, J. Nutr. Sci., 2016, 5, e47.
2. E. G. Schijlen, C. R. De Vos, A. J. van Tunen and A. G. Bovy, Modification of flavonoid biosynthesis in crop plants, Phytochemistry, 2004, 65, 2631–2648.
3. S. Kumar and A. K. Pandey, Chemistry and biological activities of flavonoids: an overview, Sci. World J., 2013, 2013, 162750.
4. E. Middleton Jr, Effect of plant flavonoids on immune and inflammatory cell function, Adv. Exp. Med. Biol., 1998, 439, 175–182.
5. D. M. Kopustinskiene, V. Jakstas, A. Savickas and J. Bernatoniene, Flavonoids as anticancer agents, Nutrients, 2020, 12, 457.
6. P. Batra and A. K. Sharma, Anti-cancer potential of flavonoids: recent trends and future perspectives, 3 Biotech, 2013, 3, 439–459.
7. A. Rauf, M. Imran, I. A. Khan, M. ur-Rehman, S. A. Gilani, Z. Mehmood and M. S. Mubarak, Anticancer potential of quercetin: a comprehensive review, Phytother. Res., 2018, 32, 2109–2130.
8. T. Okamoto, Safety of quercetin for clinical application, Int. J. Mol. Med., 2005, 16, 275–278.
9. X. Wang, Y. Yang, Y. An and G. Fang, The mechanism of anticancer action and potential clinical use of kaempferol in the treatment of breast cancer, Biomed. Pharmacother., 2019, 117, 109086.
10. T. Rengarajan and N. S. Yaacob, The flavonoid fisetin as an anticancer agent targeting the growth signaling pathways, Eur. J. Pharmacol., 2016, 789, 8–16.
11. K. P. Devi, T. Rajavel, S. Habtemariam, S. F. Nabavi and S. M. Nabavi, Molecular mechanisms underlying anticancer effects of myricetin, Life Sci., 2015, 142, 19–25.
12. L. McDonald, B. Liu, A. Taraboletti, K. Whiddon, L. P. Shriver, M. Konopka, Q. Liu and Y. Pang, Fluorescent flavonoids for endoplasmic reticulum cell imaging, J. Mater. Chem. B, 2016, 4, 7902–7908.
13. L. S. Lazarus, C. T. Dederich, S. N. Anderson, A. D. Benninghoff and L. M. Berreau, Flavonol-based carbon monoxide delivery molecule with endoplasmic reticulum, mitochondria, and lysosome localization, ACS Med. Chem. Lett., 2022, 13, 236–242.
14. Y. Shi, S. Wang, J. Wu, X. Jin and J. You, Pharmaceutical strategies for endoplasmic reticulum-targeting and their prospects of application, J. Controlled Release, 2021, 329, 337–352.
15. X. Bi, B. Liu, L. McDonald and Y. Pang, Excited-state intramolecular proton transfer (ESIPT) of fluorescent flavonoid dyes: A close look by low temperature fluorescence, J. Phys. Chem. B, 2017, 121, 4981–4986.
16. M. Tian, J. Sun, Y. Tang, B. Dong and W. Lin, Discriminating live and dead cells in dual-color mode with a two-photon fluorescent probe based on ESIPT mechanism, Anal. Chem., 2018, 90, 998–1005.
17. T. C. Johnstone, K. Suntharalingam and S. J. Lippard, The next generation of platinum drugs: targeted Pt(II) agents, nanoparticle delivery, and Pt(IV) prodrugs, Chem. Rev., 2016, 116, 3436–3486.
18. M. D. Hall, H. R. Mellor, R. Callaghan and T. W. Hambley, Basis for design and development of platinum(IV) anticancer complexes, J. Med. Chem., 2007, 50, 3403–3411.
19. D. Gibson, Platinum(IV) anticancer prodrugs–hypotheses and facts, Dalton Trans., 2016, 45, 12983–12991.
20. A. Lopez-Sanchez and H. Bertrand, Pt(IV) anticancer prodrugs bearing an oxaliplatin scaffold: what do we know about their bioactivity?, Inorg. Chem. Front., 2024, 11, 1639–1667.
21. J. J. Wilson and S. J. Lippard, Synthetic methods for the preparation of platinum anticancer complexes, Chem. Rev., 2014, 114, 4470–4495.
22. F. Navas, A. Chocarro-Calvo, P. Iglesias-Hernández, P. Fernández-García, V. Morales, J. M. García-Martínez, R. Sanz, A. De la Vieja, C. García-Jiménez and R. A. García-Muñoz, Promising anticancer prodrugs based on Pt(IV) complexes with bis-organosilane ligands in axial positions, J. Med. Chem., 2024, 67, 6410–6424.
23. O. O. Krasnovskaya, R. A. Akasov, D. V. Spector, K. G. Pavlov, A. A. Bubley, V. A. Kuzmin, A. A. Kostyukov, E. V. Khaydukov, E. V. Lopatukhina, A. S. Semkina, K. Yu. Vlasova, S. A. Sypalov, A. S. Erofeev, P. V. Gorelkin, A. N. Vaneev, V. N. Nikitina, D. A. Skvortsov, D. A. Ipatova, D. M. Mazur, N. V. Zyk, D. A. Sakharov, A. G. Majouga and E. K. Beloglazkina, Photoinduced reduction of novel dual-action riboplatin Pt(IV) prodrug, ACS Appl. Mater. Interfaces, 2023, 15, 12882–12894.
24. J. Sánchez-Camacho, S. Infante-Tadeo, A. C. Carrasco, S. Scoditti, Á. Martínez, F. Barroso-Bujans, E. Sicilia, A. M. Pizarro and L. Salassa, Flavin-conjugated Pt(IV) anticancer agents, Inorg. Chem., 2023, 62, 5644–5651.
25. S. Jin, N. Muhammad, Y. Sun, Y. Tan, H. Yuan, D. Song, Z. Guo and X. Wang, Multispecific platinum(IV) complex deters breast cancer via interposing inflammation and immunosuppression as an inhibitor of COX-2 and PD-L1, Angew. Chem., 2020, 132, 23513–23521.
26. G. Thiabaud, G. He, S. Sen, K. A. Shelton, W. B. Baze, L. Segura, J. Alaniz, R. Munoz Macias, G. Lyness, A. B. Watts, H. M. Kim, H. Lee, M. Y. Cho, K. S. Hong, R. Finch, Z. H. Siddik, J. F. Arambula and J. L. Sessler, Oxaliplatin Pt(IV) prodrugs conjugated to gadolinium-texaphyrin as potential antitumor agents, Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 7021–7029.
27. G. Pelosi, M. Ravera, E. Gabano, F. Fregonese and D. Osella, Unprecedented one-pot synthesis of an unsymmetrical cisplatin-based Pt(IV)-acetamidato complex, Chem. Commun., 2015, 51, 8051–8053.
28. M. Ravera, E. Gabano, I. Zanellato, F. Fregonese, G. Pelosi, J. A. Platts and D. Osella, Antiproliferative activity of a series of cisplatin-based Pt(IV)-acetylamido/carboxylato prodrugs, Dalton Trans., 2016, 45, 5300–5309.
29. Q. Zhou, S. Chen, Z. Xu, G. Liu, S. Zhang, Z. Wang, M. K. Tse, S. M. Yiu and G. Zhu, Multitargeted platinum(IV) anticancer complexes bearing pyridinyl ligands as axial leaving groups, Angew. Chem., Int. Ed., 2023, 62, e202302156.
30. J. Huang, W. Ding, X. Zhu, B. Li, F. Zeng, K. Wu, X. Wu and F. Wang, Ligand evolution in the photoactivatable platinum(IV) anticancer prodrugs, Front. Chem., 2022, 10, 876410.
31. N. A. Kratochwil, M. Zabel, K.-J. Range and P. J. Bednarski, Synthesis and X-ray crystal structure of trans,cis-[Pt(OAc)₂I₂(en)]: a novel type of cisplatin analog that can be photolyzed by visible light to DNA-binding and cytotoxic species in vitro, J. Med. Chem., 1996, 39, 2499–2507.
32. F. S. Mackay, J. A. Woods, H. Moseley, J. Ferguson, A. Dawson, S. Parsons and P. J. Sadler, A photoactivated trans-diammine platinum complex as cytotoxic as cisplatin, Chem. – Eur. J., 2006, 12, 3155–3161.
33. L. Brancaleon and H. Moseley, Laser and non-laser light sources for photodynamic therapy, Lasers Med. Sci., 2002, 17, 173–186.
34. N. J. Farrer, J. A. Woods, L. Salassa, Y. Zhao, K. S. Robinson, G. Clarkson, F. S. Mackay and P. J. Sadler, A potent trans-diimine platinum anticancer complex photoactivated by visible light, Angew. Chem., 2010, 122, 9089–9092.
35. H. Yao, S. Chen, Z. Deng, M.-K. Tse, Y. Matsuda and G. Zhu, BODI-Pt, a green-light-activatable and carboplatin-based platinum(IV) anticancer prodrug with enhanced activation and cytotoxicity, Inorg. Chem., 2020, 59, 11823–11833.
36. Z. Wang, N. Wang, S.-C. Cheng, K. Xu, Z. Deng, S. Chen, Z. Xu, K. Xie, M.-K. Tse, P. Shi, H. Hirao, C.-C. Ko and G. Zhu, Phorbiplatin, a highly potent Pt(IV) antitumor prodrug that can be controllably activated by red light, Chem, 2019, 5, 3151–3165.
37. Z. Deng, H. Li, S. Chen, N. Wang, G. Liu, D. Liu, W. Ou, F. Xu, X. Wang, D. Lei, P.-C. Lo, Y. Li, J. Lu, M. Yang, M.-L. He and G. Zhu, Near-infrared-activated anticancer platinum(IV) complexes directly photooxidize biomolecules in an oxygen-independent manner, Nat. Chem., 2023, 15, 930–939.
38. M. Imran, W. Ayub and I. S. Butler, Photoactivated platinum-based anticancer drugs, Coord. Chem. Rev., 2018, 376, 405–429.
39. Z. Deng, N. Wang, Y. Liu, Z. Xu, Z. Wang, T.-C. Lau and G. Zhu, A photocaged, water-oxidizing, and nucleolus-targeted Pt(IV) complex with a distinct anticancer mechanism, J. Am. Chem. Soc., 2020, 142, 7803–7812.
40. H. Shi, R. C. Marchi and P. J. Sadler, Advances in the design of photoactivatable metallodrugs: excited state metallomics, Angew. Chem., Int. Ed., 2025, 64, e202423335.
41. F. S. Mackay, S. A. Moggach, A. Collins, S. Parsons and P. J. Sadler, Photoactive trans ammine/amine diazido platinum(IV) complexes, Inorg. Chim. Acta, 2009, 362, 811–819.
42. F. Juliá, Ligand-to-metal charge transfer (LMCT) photochemistry at 3d-metal complexes: an emerging tool for sustainable organic synthesis, ChemCatChem, 2022, 14, e202200916.
43. H. C. Tai, Y. Zhao, N. J. Farrer, A. E. Anastasi, G. Clarkson, P. J. Sadler and R. J. Deeth, A computational approach to tuning the photochemistry of platinum(IV) anticancer agents, Chem. – Eur. J., 2012, 18, 10630–10642.
44. H. Kato and H. Nishitoh, Stress responses from the endoplasmic reticulum in cancer, Front. Oncol., 2015, 5, 93.
45. S. Zeng, C. Chen, L. Zhang, X. Liu, M. Qian, H. Cui, J. Wang, Q. Chen and X. Peng, Activation of pyroptosis by specific organelle-targeting photodynamic therapy to amplify immunogenic cell death for anti-tumor immunotherapy, Bioact. Mater., 2023, 25, 580–593.
46. J. Shi, W. Gao and F. Shao, Pyroptosis: gasdermin-mediated programmed necrotic cell death, Trends Biochem. Sci., 2017, 42, 245–254.
47. X. Shen, H. Wang, C. Weng, H. Jiang and J. Chen, Caspase 3/GSDME-dependent pyroptosis contributes to chemotherapy drug-induced nephrotoxicity, Cell Death Dis., 2021, 12, 186.
48. H. Yan, B. Luo, X. Wu, F. Guan, X. Yu, L. Zhao, X. Ke, J. Wu and J. Yuan, Cisplatin induces pyroptosis via activation of MEG3/NLRP3/caspase-1/GSDMD pathway in triple-negative breast cancer, Int. J. Biol. Sci., 2021, 17, 2606.
49. R. Zhang, C. Wang, Y. Guan, X. Wei, M. Sha, M. Yi, M. Jing, M. Lv, W. Guo, J. Xu, Y. Wan, X.-M. Jia and Z. Jiang, Manganese salts function as potent adjuvants, Cell. Mol. Immunol., 2021, 18, 1222–1234.

---