Colorectal cancer (CRC) has a poor prognosis and urgently needs better therapeutic approaches. 5-Aminolevulinic
acid (ALA) induced
protoporphyrin IX (
PpIX) based
photodynamic therapy (
PDT) is already used in the clinic for several
cancers but not yet well investigated for CRC. Currently, systemic administration of ALA offers a limited degree of tumour selectivity, except for intracranial tumours, limiting its wider use in the clinic. The combination of effective ALA-
PDT and
chemotherapy may provide a promising alternative approach for CRC treatment. Herein,
theranostic Ag2S
quantum dots (AS-2MPA) optically trackable in near-infrared (NIR), conjugated with
endothelial growth factor receptor (EGFR) targeting
Cetuximab (Cet) and loaded with ALA for
PDT monotherapy or ALA/
5-fluorouracil (
5FU) for the combination
therapy are proposed for enhanced treatment of EGFR(+) CRC. AS-2MPA-Cet exhibited excellent targeting of the high EGFR expressing cells and showed a strong intracellular signal for NIR optical detection in a comparative study performed on SW480, HCT116, and HT29 cells, which exhibit high, medium and low EGFR expression, respectively. Targeting provided enhanced uptake of the ALA loaded nanoparticles by strong EGFR expressing cells and formation of higher levels of
PpIX. Cells also differ in their efficiency to convert ALA to
PpIX, and SW480 was the best, followed by HT29, while HCT116 was determined as unsuitable for ALA-
PDT. The therapeutic efficacy was evaluated in 2D cell cultures and 3D spheroids of SW480 and HT29 cells using AS-2MPA with either electrostatically loaded,
hydrazone or
amide linked ALA to achieve different levels of pH or
enzyme sensitive release. Most effective
phototoxicity was observed in SW480 cells using AS-2MPA-ALA-electrostatic-Cet due to enhanced uptake of the particles, fast ALA release and effective ALA-to-
PpIX conversion. Targeted delivery reduced the effective ALA concentration significantly which was further reduced with codelivery of
5FU. Delivery of ALA via covalent linkages was also effective for
PDT, but required a longer incubation time for the release of ALA in therapeutic doses.
Phototoxicity was correlated with high levels of
reactive oxygen species (ROS) and apoptotic/necrotic cell death. Hence, both AS-2MPA-ALA-Cet based
PDT and AS-2MPA-ALA-Cet-5FU based chemo/
PDT combination
therapy coupled with strong NIR tracking of the nanoparticles demonstrate an exceptional
therapeutic effect on CRC cells and excellent potential for synergistic multistage tumour targeting
therapy.