Bioorthogonal Photocatalytic Reactions of Flavins Converting PtIV Substrates into PtII Anticancer Drugs

An anti-cancer drug that is successful at eliminating rapidly producing cells directly targets the location of the tumor and is also non-toxic until it reaches the diseased area would be the ideal. For many, Platinum-II (PtII) based chemotherapy drugs have proven life-changing, with the most commonplace drug, Cisplatin, offering treatment and cures for cancerous diseases in the breast, bladder, lung, cervix, ovaries, and neck and head, to name just some.

In the active drug form, PtII binds to DNA which disrupts cellular replication, destroying them quickly proliferating cancerous cells. However, PtII also kills healthy cells, resulting in serious side effects, particularly in those who already have kidney problems.

Non-active (prodrug) PtIV equivalents display very little to no toxicity to normal cells, and are able to be converted to the active PtII drugs through catalysis; fully comprehending these biological mechanisms and employing directed light-based therapies may offer new treatment methods for curing cancer.

To achieve this, researchers must first ascertain the catalytic processes and intermediary states present in the Pt reduction process; the FLS1000 Luminescence Spectrometer and the Edinburgh Instruments LP980 Transient Absorption Spectrometer are ideally matched to improve and measure new therapeutics (Figure 1).

(Top) The Edinburgh Instruments LP980 Transient Absorption Spectrometer and (Bottom) the FLS1000 Photoluminescence Spectrometer.

Figure 1. (Top) The Edinburgh Instruments LP980 Transient Absorption Spectrometer and (Bottom) the FLS1000 Photoluminescence Spectrometer.

Research

A group of scientists from Italy and Spain, led by Prof. Luca Salassa of the Donostia International Physics Center (San Sebastián, Spain), in partnership with Dr. Virginia Martínez-Martínez from University of the Basque Country (Bilbao, Spain), utilized Edinburgh Instruments time-resolved luminescence and transient absorption spectrometers to investigate the photocatalytic mechanisms of flavins (FLs) converting PtIV prodrug complexes to active PtII drug substrates in a biological environment.

Although metal complexes are not generally known as being substrates in catalytic reactions, the researchers were able to show how flavins are successful redox photocatalysts for these PtIV prodrug substrates, and demonstrate that the flavin’s photogenerated triplet state drives this catalytic process.

(Left) The photocatalytic mechanism of PtIV-to-PtII reduction mediated by a photoexcited flavin (FL) in the triplet state having extracted two electrons from NADH, and (Right) structures of one of the flavins, flavin mononucleotide (FMN), and one of the PtIV prodrugs (1) used in this study.

Figure 2. (Left) The photocatalytic mechanism of PtIV-to-PtII reduction mediated by a photoexcited flavin (FL) in the triplet state having extracted two electrons from NADH, and (Right) structures of one of the flavins, flavin mononucleotide (FMN), and one of the PtIV prodrugs (1) used in this study.

Figure 2 shows the photocatalytic cycle of flavins reducing PtIV prodrug substrates to active PtII drug substrates. Photoexcitation into the flavin absorption band produces an excited singlet state that quickly undergoes intersystem crossing to an excited triplet state, 3FL*. Because of the huge oxidizing potential of 3FL*, it can extract two electrons from biological donors such as NADH, to produce the active catalytic species HFL– (or H2FL depending on the PH of the solution). HFL– can reduce the PtIV prodrug substrates to active PtII drug substrates and then repeat the cycle.

Without light, this process can still go on, but its efficiency is cut substantially. To verify the process, the researchers utilized absorption spectroscopy of flavin mononucleotide (FMN) with NADH in an oxygen free environment following light irradiation; this demonstrated the characteristic absorption change to HFMN-. Additional H1 NMR studies of NADH, FMN, and PtIV substrate 1 (Figure 1) after light irradiation signaled the presence of HFMN– and its return to FMN when PtIV substrate 1 is added to the solution.

(Top) The time-resolved transient absorption spectrum using an ICCD camera of 3FMN*, and (Bottom) the lifetime traces using a PMT of the triplet state measured at 700 nm for FMN, FMN + NADH, and FMN + NADH + PtIV substrate 1 showing significant quenching of the native 3FMN* lifetime. The excitation wavelength was 445 nm for all measurements.

Figure 3. (Top) The time-resolved transient absorption spectrum using an ICCD camera of 3FMN*, and (Bottom) the lifetime traces using a PMT of the triplet state measured at 700 nm for FMN, FMN + NADH, and FMN + NADH + PtIV substrate 1 showing significant quenching of the native 3FMN* lifetime. The excitation wavelength was 445 nm for all measurements.

To further prove this photocatalytic mechanism’s intermediate states, the researchers skillfully utilized nanosecond transient absorption to measure the photoexcited FMN spectral and lifetime information on its own, and then when combined with NADH and PtIV substrate 1 (Figure 3). A broad, high oscillator strength NIR triplet absorption can be directly linked to the formation of  3FMN*, whose lifetime is pointedly quenched from 14 μs to approximately 2.4 μs where NADH and PtIV substrate 1 are added. This data makes it evident that the 3FMN* triplet state is generated in high yield and is the primary photoproduct needed to start the catalytic cycle. To guarantee that the singlet state has limited, if any, involvement in the photocatalytic process, photoluminescence measurements of the spectra and associated lifetimes for FMN, NADH, and PtIV substrate 1 were measured.

Fluorescence lifetimes of FMN, FMN + NADH, and FMN + NADH + PtIV substrate 1, showing no quenching changes to the singlet lifetime of 4.7 ns for the native FMN. Excitation wavelength was 445 nm and the emission wavelength was monitored at 540 nm, the emission peak of FMN.

Figure 4. Fluorescence lifetimes of FMN, FMN + NADH, and FMN + NADH + PtIV substrate 1, showing no quenching changes to the singlet lifetime of 4.7 ns for the native FMN. The excitation wavelength was 445 nm and the emission wavelength was monitored at 540 nm, the emission peak of FMN.

Figure 4 illustrates the fluorescence decay profiles of FMN, FMN + NADH, and FMN + NADH + PtIV substrate 1; no lifetime changes were seen from the native FMN’s 4.7 ns lifetime upon either two other molecules being added. This leads to the conclusion that neither NADH nor PtIV substrate 1 interact with the photoexcited singlet state, 1FMN*, and that the photocatalytic process starts only upon the formation of the 3FMN* following intersystem crossing from the singlet state.

Conclusion

Utilizing photocatalytic processes is just one of many instruments on hand for medicinal chemists working on revolutionary new drug molecular systems, treatments, and deliveries. Edinburgh Instruments LP980 Transient Absorption and FLS1000 Photoluminescence Spectrometers are designed to offer the best measurements for researchers to study and understand the underlying photocatalytic mechanisms, easing the way for this exciting research to reach the highest levels.

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.

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