Inclusion Criteria:  Healthy male or female volunteer, aged 18-60 years. BMI (Quetelet index) in the range 18.0-30.9 kg/m2. Able to understand the nature of the trial and any hazards of participating in it. Ability to communicate satisfactorily with the investigator and to participate in, and comply with the requirements of, the entire trial. Willingness to give written consent to participate after reading the information and consent form, and after having the opportunity to discuss the trial with the investigator or their delegate. Agree to follow the contraception requirements of the trial Agree not to donate blood or blood products during the study and for up to 3 months after the administration of the trial medication. Spirometry readings (FEV1 and FVC) to be ≥ 80% of predicted value at the screening visit, calculated using National Health and Nutrition Examination Survey (NHANES) reference. If a subject's FEV1 or FVC is outside that range at the screening visit, the test may be repeated once on another day. Registered with a General Practitioner (GP) in the UK (Part A only). Willingness to give written consent to have data entered into The Overvolunteering Prevention System (TOPS).  Exclusion Criteria:  Woman who is pregnant or lactating, or pre-menopausal woman who is sexually active and not using a reliable method of contraception. Clinically relevant abnormal medical history, physical findings, ECG, or laboratory values at the pre-trial screening assessment that in the opinion of the investigator could interfere with the objectives of the trial or the safety of the volunteer. Presence or history of acute or chronic illness sufficient to invalidate the volunteer's participation in the trial or make it unnecessarily hazardous. Presence or history of respiratory disease, including (but not limited to) asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis, emphysema, requiring acute or chronic medication. Presence of nasal polyps or significant nasal abnormalities. Symptoms of respiratory illness (including, but not limited to, runny nose, sore throat, sneezing, coughing or wheezing) at the screening visit or before dosing on Day 1. Tympanic temperature > 37.5°C at the screening visit or before dosing on Day 1. Impaired endocrine, thyroid, hepatic, respiratory or renal function, diabetes mellitus, coronary heart disease, or history of any psychotic mental illness. History of psychiatric disease, as determine by the investigator. History or presence of malignant disease. Immune-suppressed status, resulting from either disease or medication, as determined by the investigator. Presence or history of severe adverse reaction to any drug or the excipients of Neumifil. Known allergy to tetracycline antibiotics. Use of a prescription medicine (except HRT in female subjects), including oral contraceptives, during the 28 days before the first dose of trial medication, or use of an over-the-counter medicine, with the exception of acetaminophen (paracetamol) and vitamin or nutritional supplements, during the 7 days before the first dose of trial medication. Receipt of an investigational product (including prescription medicines) as part of another clinical trial within the 3 months before admission to this study; in the follow-up period of another clinical trial at the time of screening for this study. Receipt of a COVID-19 vaccine within 7 days before the first dose of trial medication, or anticipate receiving a COVID-19 vaccine within the 7 days after a (final) dose of trial medication. Presence or history of drug or alcohol abuse, or regular intake of more than 14 units of alcohol weekly. Use of cigarettes or nicotine-containing products during the 6 months before first dose of trial medication. Blood pressure and heart rate in supine position at the screening examination outside the ranges: blood pressure 90-140 mm Hg systolic, 40-90 mm Hg diastolic; heart rate 40-100 beats/min. Triplicate measurements will be made (at least 2 min apart), and a mean value outside the above ranges will lead to exclusion. Repeat measurements (in triplicate) are permitted if values are borderline (ie values that are within 5 mm Hg for blood pressure or 5 beats/min for heart rate) or if requested by the investigator. Subjects can be included if the repeat value is within range or still borderline but deemed not clinically significant by the investigator.  QTcF value, of > 450 msec (men) or > 470 msec (women); or QRS duration  ≥ 120 msec, measured on 12-lead ECG at the screening visit. Triplicate measurements will be made, and a mean value used to determine eligibility. A repeat (in triplicate) is also allowed on one occasion for determination of eligibility.  Possibility that the volunteer will not cooperate with the requirements of the protocol.  Positive test for hepatitis B surface antigen, hepatitis C or HIV. NOTE:  participants with positive hepatitis C antibody owing to resolved disease can be included only if a hepatitis C ribonucleic acid (RNA) test is negative.  Positive test for SARS-CoV-2 (polymerase chain reaction; PCR) or suspected exposure to the SARS-CoV-19 virus during the 14 days before screening. Loss of more than 400 mL blood during the 3 months before the trial, eg as a blood donor. Objection by GP to volunteer entering trial.