The goal of this phase II/III randomized clinical trial is to evaluate the effect of adding rectal ozone therapy to the usual management of patients with paresthesia (numbness and/or tingling) due to chemotherapy-induced peripheral neuropathy (CIPN). Ozone treatment consists of the rectal insufflation of 180 - 300 milliliters of an ozone/oxygen gas mixture.

The main questions to answer are:

1. Can ozone therapy improve patients' self-perceived level of numbness and tingling?
2. Can ozone therapy improve patients' self-perceived health-related quality of life (HRQoL)?

In 42 patients with chronic numbness and tingling secondary to chemotherapy, the researchers will compare:

• the addition of rectal ozone insufflations
• versus the addition of rectal oxygen insufflations (placebo). Participants will receive 40 rectal gas (ozone versus oxygen) insufflations in 16 weeks and will continue other symptomatic or cancer treatments prescribed by their oncologists.

Before treatment, after treatment, and 12 weeks after treatment, they will be evaluated:

• Several questionnaires about neuropathy, quality of life, and anxiety and depression.
• Biochemical parameters of oxidative stress and inflammation
• Hyperspectral images of hands and feet
• Toxicity of procedure.

Rationale Chemotherapy-induced peripheral neuropathy (CIPN) can lead to a decrease and/or interruption of chemotherapy treatment, limiting its efficacy and decreasing patients' quality of life. Therapeutic measures for CIPN are very limited in number and efficacy. Our previous experience has suggested the potential clinical usefulness of adjuvant treatment with ozone in patients with CIPN. The hypothesis of the trial is that ozone treatment will improve numbness and tingling symptoms in patients with CIPN.

Primary objectives:

To evaluate the effect of adding ozone to the usual management of patients with paresthesia (numbness and/or tingling) due to chemotherapy-induced peripheral neuropathy (CIPN), Grade 2 (moderate symptoms and/or limitation in instrumental activities of daily living) or higher, on:

1. patients' self-perceived level of paresthesia
2. patients' self-perceived health-related quality of life (HRQoL).

Secondary objectives:

To evaluate (in patients with numbness and/or tingling secondary to CIPN) the effect of adding ozone to the usual management on:

1. the additional direct costs incurred in the application of ozone and evaluate the cost-effectiveness of the administration of ozone in these patients in comparison with the usual exclusive treatment.
2. the evolution of the sensory neuropathy
3. the evolution of the level of anxiety and depression,
4. the evolution of biochemical parameters related to oxidative stress and chronic inflammation.
5. the evolution of hyperspectral signatures obtained from hands and feet.
6. to evaluate the toxicity of rectal ozone treatment in these patients.

Main trial endpoints.

1. Percentage of change from baseline in "numbness and tingling" self-perceived by patients at the end of follow-up (week 28 after the commencement of ozone treatment)
2. Change from baseline in "quality of life" (using the EQ-5D-5L questionnaire) self-perceived by patients at the end of follow-up (week 28 after the commencement of ozone treatment)

Secondary trial endpoints.

To evaluate the percentage of change from baseline in all the secondary objectives:

• at the end of ozone treatment (weeks 16)
• and at the end of follow-up (week 28)

Trial design. Phase II-III randomized triple-blind clinical trial. The duration of each patient in the study will be 28 weeks: 16 weeks of treatment and 12 weeks of follow-up. The planned total duration of the project is 60 months.

Trial population. 42 adult patients (>= 18 years old), with any tumor, with paresthesias (numbness and/or tingling) due to CIPN, grade of toxicity >= 2 (according to the Common Toxicity Criteria for Adverse Events (CTCAE) from the National Cancer Institute of EEUU, v.5.0), for >= 3 months.

Intervention.

All patients will receive the usual management and treatment for their symptoms + "40 sessions of rectal insufflation of O3/O2 gas mixture" in 16 weeks (3 or 2 sessions per week):

• Ozone group, O3/O2 concentration increasing from 10 to 30 µg/mL (µg of O3 by mL of O2).
• Control-placebo group, O3/O2 concentration = 0 µg/mL (this is: only O2).

Inclusion Criteria:

• 1. Adults > = 18 years old.
• 2. Previous treatment with any chemotherapy because of any tumor.
• 3. Clinical diagnosis of paresthesia (numbness, tingling) secondary to CIPN, with toxicity Grade > = 2 (according to the Common Toxicity Criteria for Adverse Events (CTCAE) from the National Cancer Institute of EEUU, v.5.0) for > = 3 months.
• 4. Without neurotoxic chemotherapy > = 3 months.
• 5. Cancer disease is stable or in remission.
• 6. Life expectancy > = 6 months.
• 7. Before enrollment, women of childbearing potential should obtain a negative result in the serum or urine pregnancy test at the screening visit and accept the use of appropriate contraceptive methods at least from 14 days before the first ozone therapy session up to 14 days after the last one.
• 8. To sign and date the study-specific informed consent

Exclusion Criteria:

• 1. Age < 18 years.
• 2. A woman who is lactating, pregnant, suspected of being pregnant, or a woman of childbearing potential who does not use adequate contraceptive methods.
• 3. Suspected symptoms are due to diabetic or compressive neuropathy.
• 4. Severe psychiatric disorders.
• 5. Inability to complete the quality of life questionnaires.
• 6. Elevation above 5 times the maximum limit of normal creatinine.
• 7. Patient who is hemodynamic or clinically unstable or who requires urgent or short-term interventional measures.
• 8. Neoplasia in progression requiring recent initiation of systemic treatment or maintenance with neurotoxic chemotherapy.
• 9. Life expectancy (for any reason) < 6 months.
• 10. Known allergy to ozone, known glucose 6 phosphate dehydrogenase (G6PD) deficiency, or hemochromatosis.
• 11. Contraindications or impossibility for rectal ozone treatment or to attend regularly to the treatment.
• 12. Not meeting each and every one of the inclusion criteria

It will be available (after request):

• Individual participant data (IPD) that underlie the results reported in further articles, after deidentification
• Data will be available after publication, ending 36 months following article publication.
• They will be available for investigators whose proposed use of the data has been approved by an independent review committee identified for this purpose.
• Study protocol

Proposals should be directed to: bernardinoclavo@gmail.com To gain access, data requestors will need to sign a data access agreement.

Data will be available after publication, ending 36 months following article publication.

Data will be available for investigators whose proposed use of the data has been approved by an independent review committee identified for this purpose.

- Study protocol

Proposals should be directed to: bernardinoclavo@gmail.com To gain access, data requestors will need to sign a data access agreement.

• the oncologists/hematologists who treat and follow the patient regularly,
• researchers who carry out biochemical determinations or functional tests,
• the staff who obtain the economic data,
• statisticians