Table 6.
Brief summary of the revised literature with some conclusions; unmet needs; future perspectives.
| Findings from literature | Conclusions | Future perspectives |
|---|---|---|
| Bladder mechanism | ||
| Radiation effect at the molecular level (direct and indirect damage to DNA) is followed by downstream abnormalities of bladder wall in three phases: 1) Early/acute phase: reversible → Increase of NF-κB → COX2 and prostaglandin expression → Vasodilatation, increased muscle tone → hyperemia, edema → increase of ICAM-1 → leucocyte infiltration → inflammatory symptoms (frequency, urgency, dysuria) 2) Symptom-free phase Late phase: persistent, fibrotic → UP-III downregulation and loss of umbrella cells → increase of permeability → chemical irritation from urine components → Increase of TGF-β1 expression → accumulation of extracellular matrix and collagen deposition → development of fibrosis → hematuria, permanent reduction of bladder compliance, voiding failure |
The full comprehension of RT-induced urothelial toxicity and the availability of models that faithfully recapitulate all the pathological paths, both early and late phases, still represent an unmet need. The establishment of reliable preclinical models mimicking urothelial toxicity is fundamental for testing more “tailored” novel therapies. |
Given the current interest in hypo-fractionation and ablative therapies, investigations mimicking extreme hypo-fractionation (e.g., radical doses delivered in 1–5 fractions) should help in better understanding the partially unknown mechanisms of bladder radiation response in these extreme situations. Experiments set to identify the mechanisms underlying “spatial effects” would be of paramount importance in possibly guiding plan optimization to selectively reduce the dose to these sub-structures. |
| Animal models and dose set up: | ||
| •Each research group developed their own animal model using different strains of rats or mice. •Radiation dose was tested in the range 5–40 Gy. •The general practice was to use a single radiation dose of 20–25 Gy, approximately equal to ED50 and estimated to mimic the delivered clinical doses to pelvic tumors. •Doses over 20 Gy proved to be associated with a higher toxicity rate and more severe symptoms. •ED50 increases with the number of fractions and the interval between fractions. •Late radiation injury seemed to be inversely related to the dose given in the first treatment and independent of the interval between treatments. |
Very different experimental settings have been used: a “standard” universally recognized RC model is still missing. Even though a single dose of 40 Gy is well above the dose delivered in a clinical setting, the use of high dose can be useful for a better understanding of the underlying biological mechanisms. |
Despite the availability of micro-irradiators with theoretically significant potentials for high-precision experiments, animal studies focused on gaining a better understanding of dose, fractionation and volume effects are largely lacking.Reliable and reproducible methods to quantify the severity of RC in animal models should be realized. |
| In vivo functional evaluation | ||
| •Acute damage is confined to only a few weeks after irradiation, irrespective of the dose delivered, with a biphasic response at about 7 and 23 days after RT, respectively. •late toxicity could emerge at different time lapses (within 6 month to 1 year) with intensity depending on radiation dose and fractionation. No changes in the diurnal urinary pattern were observed during cystometry. If male mice/rats are used, a surgical implantation of the catheter is deemed necessary. |
Cystometry is the “state-of-the- art” objective tool in evaluating the in vivo response to radiation damage, in terms of reservoir function and/or micturition frequency. | High-quality pre-clinical imaging platforms are expected to extend the potential of non-invasive assessment of RC severity. |
| Histopathological model of RC | ||
| •H&E (the most informative): recognition of both early acute and late histological changes. •Masson trichrome: to assess the level of bladder wall fibrosis as an intensity-based score or as a percentage of bladder wall area score. •urothelial and inflammation markers to better visualize the urothelium (e.g., COX-1/2 and UP-III). •Simple “positive vs. negative” staining using integrated optical density: to better visualize urothelium loss and loss of smooth muscle. |
IHC is the gold standard for the tracking of disease progression in preclinical models. There are limitations in the application of this knowledge to humans that must be considered when planning clinical trials and experimental therapies. |
The interaction between radiation induced reactions, damage repair and the immune system in the case of combined immune-radiotherapy is an extremely promising field of investigation, possibly involving several pelvic tumors. |
| Radioprotective agents | ||
|
Clinical management of RC: •Systemic treatments (e.g., anticholinergic agents and β3-adrenergic receptor agonists, TCDO, SPP, tranexamic acid): non-invasive and circumvent inpatient hospital admission; these therapies suffer from a very low efficacy, often accompanied by dose-dependent toxicity. •Local treatments and bladder irrigation: considered the first line of intervention in all grades of the disease, aiming at sterilization, arrest of focal bleeding points and removal of blood clots (e.g., intravesical therapies). •hyperbaric oxygen and laser ablation: emerged as non-invasive management; they are, however, cumbersome for patients requiring lengthy treatments, and a level of fitness that many patients with radiation cystitis do not possess.Classes of RAs: •agents for the prophylaxis of RT injuries, administered before exposure. •mitigators given during or shortly after RT, aimed at minimizing or preventing the effects of radiation on cells/tissues. •treatments or therapeutic preparations applied after RT, to ameliorate radio-induced symptoms.Compounds in radioprotection preclinical research: •Bortezomib: implicated in the NF-κB blockade. •Hormone relaxin: reversing fibrosis. •Tacrolimus and L-arginine: to hinder the production and release of pro-inflammatory cytokines. |
RAs improve the range of clinical options for the management of the RT-induced toxicity in combined therapies. | We must expect the translation of pre-clinical results into clinical trials testing the protective effects of RAs, especially in situations where high doses need to be delivered (e.g., prostate cancer) and for patients at higher risk of toxicity due to genetic predisposition or clinical factors (e.g., the impaired baseline urinary function of patients irradiated after prostatectomy). Future goals will be the identification of novel molecules and strategies to pursue either alone or in combination in order to guarantee a broader efficacy at a cellular, tissue, organ and whole organism level. |
NF-κB, nuclear factor-kappa B; COX2, cyclooxygenase; ICAM-1, intercellular adhesion molecule 1; UP-III, uroplakin 3; TGF-β1, transforming growth factor beta-1; ED50, radiation dose producing the damage in 50% of animals; RC, radiation cystitis; reservoir function, reduction in the bladder capacity by >50% at a fixed intravesical pressure; H&E, hematoxylin and eosin; IHC, immunohistochemistry; RA, radioprotective agent; TCDO, Tetrachlorodecaoxygen; SPP, sodium pentosan polysulphate.