Page 15 - Read Online

Brown. Rare Dis Orphan Drugs J. 2025;4:21 https://dx.doi.org/10.20517/rdodj.2025.14 Page 11 of 15

and granulosum and a higher proportion of apocrine versus eccrine sweat glands, potentially affecting
topical drug diffusion. Indeed, an in vitro porcine skin permeability assay of a lipophilic compound
predicted to have high skin permeability revealed striking differences: no permeability in porcine skin
[79]
compared with human skin . These physiologic distinctions could influence outcomes and dose
determination in preclinical trials of topical medications. The skin of juvenile minipigs best replicates adult
[80]
human skin ; however, CNs in minipig models arise in adulthood, when their skin is less biosimilar to that
of humans. Thus, these models may be more appropriate for testing systemic therapies rather than topical
drugs. Further basic research is needed to understand why murine and porcine models fail to spontaneously
and accurately replicate CN morphology and distribution.

ECM is the major component of CNs and can block drug delivery by increasing tumor parenchymal back
pressure against capillary extravasation and physically impeding large molecule diffusion. Additionally, the
ECM may interfere with T cell and antibody-based therapies, as cytotoxic T cells must navigate this dense
“briar patch” of ECM to reach their targets. Collagen turnover is tightly regulated through an interplay
between extracellular zinc-dependent endopeptidase activity and intracellular recycling. MMPs initiate
large-scale collagen breakdown by cleaving a small moiety of extracellular collagen after it binds to cell
surface receptors. Internalization then occurs through phagocytosis, pinocytosis, endocytosis, or
autophagocytosis, with subsequent degradation mainly within phagolysosomes by cathepsin cysteine
proteases. While current CN-directed therapies aim to kill or induce senescence in tumor cells, the bulk of
cutaneous neurofibromas is composed of ECM. Extracellular collagen, in the absence of enzymatic
degradation by MMPs, has a longevity that dwarfs the average human life expectancy due to its dense helical
structure and resistance to proteolysis. In CN tumor cells, MMP expression is downregulated in a
Ras-independent manner via aryl hydrocarbon receptor-induced extracellular signal-regulated kinase (ERK)
phosphorylation. Therefore, blocking Ras signaling with MEK inhibitors is not expected to restore normal
collagen turnover. Hypothetically, even if all tumor cells within CNs were eliminated, the skin bumps
themselves might remain.

Future strategies should consider a dual-targeted or stepwise approach to address both the tumor cells and
the persistent ECM. Potential strategies include the injection of ECM-degrading enzymes or systemic
treatment with small molecules that upregulate lysosomal collagen degradation, either preceding or
concurrent with Ras pathway inhibition. More attention should be given to the role of the ECM in
drug-resistant tumors such as CNs, including studies on diffusion parameters in intact tumors to inform
pharmacotherapy development, and investigations into the tumor’s internal pressure dynamics that could
reduce drug penetration, especially within hypoxic and acidotic tumor cores. Targeting the ECM may not
only improve treatment outcomes but could also serve as a biomarker of treatment efficacy. Annexin A2
mediates collagen secretion and subsequent adhesion to the basement membrane and could potentially be
targeted to prevent COLVI deposition within CNs. Lysosome activators could also be evaluated as a means
to expedite collagen digestion. C6M, a metabolite of COLVI degradation, can be detected in serum and
might serve as a biomarker for the global response to CN-directed therapies targeting the ECM.

CONCLUSION
Developing new and better treatments for CNs is a major priority for the 0.03% of the global population
affected by NF1. CNs induce anxiety and depression, negatively impact careers and relationships, cause pain
and pruritus, and present a constant visual reminder of a relentlessly progressive disease despite our best
current treatments. Fortunately, the NF research community has listened to patient demands, dedicating
funding and focus to addressing CNs. Alongside the laborious development of pharmacotherapies and the
continued refinement of animal models, access to simple surgical resections could be expanded by