| 1 |
Heme biosynthesis loading to PpIX accumulation |
↑ PpIX (often higher and more spatially heterogeneous) |
↑ PpIX (typically lower; tissue-dependent) |
R |
Fluorescence contrast prerequisite for PDD/FGS and PDT substrate formation |
Tumor selectivity is largely metabolic (porphyrin pathway flux, transporter expression, ferrochelatase activity, iron availability); not a receptor-targeted drug in the classic sense. |
| 2 |
ROS photogeneration by excited PpIX |
↑ ROS (requires light + O₂) |
↑ ROS (requires light + O₂) |
P |
Type II (singlet oxygen) and Type I oxidative damage driving phototoxicity |
This is the dominant “killing” lever for PDT; absent illumination, ROS burst is not the main mode. |
| 3 |
Mitochondria / MPTP |
↓ mitochondrial function; ↑ MPTP (context-dependent) |
↓ mitochondrial function; ↑ MPTP (context-dependent) |
P |
Energetic collapse and amplification of cell-death signaling |
PpIX can localize to mitochondria in many settings; mitochondrial photodamage is a common execution node for PDT. |
| 4 |
Cell-death programs |
↑ apoptosis/necrosis (dose-dependent) |
↑ apoptosis/necrosis (dose-dependent) |
R |
Tumor cell kill and lesion clearance |
Phenotype depends on light dose-rate, oxygenation, subcellular PpIX localization, and baseline stress defenses. |
| 5 |
Vascular injury and perfusion failure |
↓ perfusion (context-dependent) |
↓ perfusion (context-dependent) |
R |
Secondary tumor control via microvascular damage |
Most relevant in vivo; magnitude depends on illumination geometry and local vascular photosensitization. |
| 6 |
NRF2 antioxidant stress response |
↑ NRF2 programs (context-dependent) |
↑ NRF2 programs (context-dependent) |
G |
Adaptive resistance to oxidative photostress |
Often a downstream consequence of photodynamic redox stress; clinically relevant as a resistance axis in repeat/low-dose contexts. |
| 7 |
Iron handling and ferrochelatase constraint |
↓ ferrochelatase constraint can yield ↑ PpIX (model-dependent) |
Variable |
R |
Controls PpIX-to-heme conversion and thus fluorescence/phototoxic substrate levels |
Iron availability and heme-pathway enzyme balance can shift PpIX accumulation; a key reason for inter-tumor variability. |
| 8 |
Chemosensitization or Radiosensitization |
↑ sensitivity (context-dependent) |
Variable |
R |
Combination leverage via oxidative injury and stress overload |
Evidence is indication- and protocol-specific; synergy is plausible but not universal and can be limited by light/oxygen constraints. |
| 9 |
Clinical Translation Constraint |
Depth-limited light delivery; oxygen dependence; heterogeneous PpIX; workflow timing; photosensitivity risk; tissue-selective illumination required |
— |
Defines where 5-ALA is clinically practical |
For most solid tumors, light penetration and geometry (and local O₂) are the hard constraints; these often dominate over “molecular pathway” considerations. |