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The Mixed Reality of PDT Selectivity

Studies consistently show that PDT selectivity is far from reliable, with many revealing “no selectivity for tumor tissue versus normal epidermis” in basal cell carcinomas after topical ALA application.

Quantitative Selectivity Ratios Found in Studies:

  • Colon cancer cells: Studies showed tumor vs normal ratios of 10:1-20:1 in cultured colon carcinoma cells, but this was attributed to differences in PPIX metabolism rather than initial ALA uptake
  • Condylomata: In 17 of 25 condylomata, there was significantly greater fluorescence compared to adjacent normal skin, with the greatest lesional to normal skin fluorescence ratios occurring after 2 hours
  • UV-induced skin tumors: Higher levels of PpIX were measured in tumors compared to normal skin, with higher tumor to normal skin PpIX fluorescence ratios measured after application of 8% and 16% ALA-Me versus 2%
  • Comparative photosensitizers: PDT selectivity at equal anti-tumor effect was ranked: Chlorin e6 > ALA-PpIX > Photofrin II, suggesting ALA-PDT falls in the middle range for selectivity

Why Selectivity Varies So Much:

The research reveals several factors affecting selectivity:

  1. Metabolic Differences: Cancer cells show accumulation of PpIX due to inactivation or downregulation of ferrochelatase, the enzyme that converts PpIX to heme
  2. Cell State Matters: Dormant cancer cells actually accumulated high PpIX levels and were more sensitive to ALA-PDT, with upregulated PEPT1 (ALA importer) and ABCB6 (porphyrin transporter) and downregulated ABCG2 (PpIX exporter)
  3. Penetration Issues: In many basal cell carcinomas, topical ALA provided little or no PpIX in deep tumor lobules, with grossly brighter external fluorescence possibly due to enhanced penetration through tumor-reactive stratum corneum

The Clinical Reality:

Despite considerable interest in ALA-PDT, “only little is known concerning the reasons for the selective accumulation of PpIX in neoplastic tissue upon ALA administration”. The 3-4× selectivity ratio mentioned in my previous post appears optimistic—many studies show either no selectivity or highly variable selectivity depending on tumor type, depth, and application method.

Skin Cancer PDT Selectivity Studies
Basal Cell Carcinoma – The Most Damning Study:
A key study examining 16 BCCs found “no selectivity for tumor tissue versus normal epidermis” after topical ALA application. In many nodular and infiltrating BCCs, topical ALA provided “little or no PpIX in deep tumor lobules” PLOSScienceDirect. The grossly brighter fluorescence over tumors was attributed to enhanced penetration through damaged stratum corneum rather than true selectivity.


UV-Induced Skin Tumors (Actinic Keratoses/SCC)
Studies in hairless mice with UV-induced skin tumors (1-2mm lesions representing actinic keratoses or squamous cell carcinomas in situ) showed “higher levels of PpIX were measured in tumors compared to normal skin”. However, the selectivity was concentration-dependent—higher tumor to normal skin PpIX fluorescence ratios were measured after application of 8% and 16% ALA-Me than after 2%.

Condylomata (HPV Skin Lesions)
In 17 of 25 condylomata, there was significantly greater fluorescence compared to adjacent normal skin, with greatest lesional to normal skin fluorescence ratios occurring after 2 hours. The mechanism was likely “enhanced stratum corneum permeability” rather than metabolic selectivity.

The Penetration Problem
Most apparent “selectivity” in skin cancers comes from barrier disruption rather than metabolic differences. Damaged skin allows better ALA penetration, creating the illusion of selectivity when it’s really just enhanced drug delivery through compromised barriers.

Clinical Implications
BCCs often show poor depth penetration and no true selectivity
Superficial lesions (AKs) may show some selectivity, but it’s marginal
Most apparent selectivity is due to barrier function differences, not metabolic targeting

Bottom Line for Skin Cancer
The selectivity that justifies PDT for skin cancers is largely a penetration artifact, not true metabolic discrimination. This makes proper protocol execution even more critical—you’re working with minimal true selectivity to begin with.

Bottom Line
The selectivity that makes PDT theoretically appealing is inconsistent and often poor in practice. When it works, it’s often due to enhanced penetration through damaged skin barriers rather than true metabolic selectivity. This explains why proper protocol execution becomes so critical—with marginal selectivity, any technical shortcuts quickly eliminate whatever therapeutic advantage exists.