Cancer is the second leading cause of death worldwide. Often patients with cancer don’t respond well to standard therapy of surgery, radiation, chemotherapy, or immunotherapy. Alternative approaches and procedures for cancer treatment are a pressing need. Mechanical therapy, the potential fifth pillar on the fight against cancer, is a new promising strategy under development to treat cancer and/or to modify biological activity with important pharmacological implications.

Our research group at Rice University has been exploring the development of molecular machines to open cell membranes mechanically, which utilized Feringa-type motors that rotates at 3 x 10⁶ rotations per second upon activation with 365 nm ultraviolet (UV) light or 405 nm visible light leading to necrotic cell death in cancer (https://doi.org/10.1038/nature23657, and https://doi.org/10.1021/acsami.9b21497) or destruction of bacterial cell walls (https://doi.org/10.1021/acsnano.9b07836 and https://doi.org/10.1126/sciadv.abm2055).

Near-infrared (NIR) light, which is in the optical therapeutic window, is ideally suited for clinical application of light-activated molecular mechanical therapies because of minimal NIR light absorption by hemoglobin and water, coupled with significant NIR light penetration through human tissue. We have previously exploited two-photon NIR light  activation of Feringa-type motors for inducing rapid cellular necrosis (https://doi.org/10.1021/acsnano.9b01556), but that technique requires large laser-generated fluxes of photons, the depth of penetration is shallow, and the area of coverage is restricted to smaller-sized domains, which is impractical for broad clinical applications.

We have discovered that single photon NIR light actuation of molecular vibronic modes, a plasmon coupled to a whole-molecule motion, in cell membrane associated aminocyanines, can be exploited to rapidly kill cells by necrosis (https://doi.org/10.1038/s41557-023-01383-y). Vibronic-driven action (VDA), a new type of molecular mechanical action, is distinct from both photodynamic therapy and photothermal therapy since its mechanical effect on the cell membrane is not inhibited by scavengers of reactive oxygen species (ROS), and it does not induce thermal killing (Figure 1). The ultrafast concerted whole-molecule oscillations of VDA-induced mechanical disruption is estimated to be 4 x 10¹³ oscillations per second, 7 orders of magnitude faster than the rotations in Feringa-type molecular motors. The molecules that destroy cell membranes through VDA have been termed molecular jackhammers (MJH).

VDA action by MJH activated by NIR light, are well suited for clinical applications. VDA-mediated mechanical destruction is achieved using very low concentrations (500 nM) of the aminocyanines or low doses of light (12 Jcm^-2, 80 mW cm^-2 for 2.5 min). By applying VDA-mediated therapy, the complete eradication of human melanoma cells was achieved in vitro (Figure 1f). The treatment of melanoma tumors resulted in 50% tumor-free efficacy in mice even after 7 months of the study.

The major challenge in this study was to distinguish mechanical action from the photothermal and photodynamic effects since cyanines are reported to be photothermal and photosensitizer agents. In addition to the evidence in Figure 1, in the full article we carefully and comprehensively present experiments to differentiate the mechanical action from other modes of action. For example, we included control cyanine molecules that have similar or higher molar extinction coefficients but with weaker VDA action than the strongest MJH. These weaker VDA action cyanines were less active in permeabilizing cell membranes even when they have a larger molar extinction coefficient, indicating that a photothermal mechanism is unlikely to be responsible for the opening of the cell membranes at the low concentrations used (<2 μM). In addition, we demonstrated that by using indocyanine green (ICG) as a photothermal control, minimal photothermal heating, ~1 °C increase, of the media occurred at a concentration of 8 μM; these conditions were insufficient to permeabilize cells. However, at 100 μM ICG we observed higher photothermal heating, ~6 °C increase, but only 13% of the cells were permeabilized by DAPI. Further increases in the concentration of ICG led to little improvement in the cell membrane permeabilization because the optical saturation of the media reduced the NIR light penetration. This collection of evidence supports our conclusion that photothermal heating is not responsible for the opening of cell membranes when using light activated MJH at low concentrations of <2 μM to ~8 µM. To distinguish the mechanical from the photodynamic effects of ROS, we compared our most active MJHs with cyanines that produce higher levels of ROS. DiR, a lipophilic cell membrane targeting cyanine and a strong photosensitizer, produces 10-fold more ROS than the most active MJH, but DiR did not permeabilize the cells when used at 2 µM. Even when the concentration of DiR was increased 20 μM and the ROS production increased 19-fold relative to the most active MJH, light activated DiR did not permeabilize the cells. To further support the proof of mechanical action, we demonstrated that light activated MJH opened and disassembled synthetic lipid vesicles composed of oxidation-resistant phospholipids (saturated phytanoyl phospholipids). An ROS-mediated mechanism was excluded from this set of experiments since saturated phytanoyl alkyl chains are oxidation resistant. Based on this evidence, we concluded that the most plausible mechanism of action for vibronic-mode activation in cyanines is by mechanical action as molecular jackhammers.