HiSNAP trial—a multicentre, randomised, open-label, blinded end point, safety and efficacy trial of conventional (300 mg/kg) versus higher doses of acetylcysteine (450 mg/kg and 600 mg/kg) in patients with paracetamol overdose in the UK: study protocol

Abstract

Introduction

In overdose, a larger proportion of paracetamol (acetaminophen) is converted in the liver to the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI). Glutathione (GSH) is the endogenous antioxidant that protects cells from NAPQI-induced injury. In overdose, GSH stores may become depleted, leaving NAPQI free to produce liver damage. N-Acetylcysteine (NAC) helps prevent paracetamol toxicity by replenishing liver GSH. This protective effect of NAC produces specific metabolites in the circulation. Currently, regardless of the paracetamol dose ingested, patients in the UK receive a dose of NAC based only on their weight. Basic pharmacology, mathematical modelling and observational studies suggest that this dose may be insufficient in some patients (particularly those taking a large overdose).

Methods and analysis

A multicentre trial, taking place across several hospitals in Scotland, UK, within Emergency Departments and Acute Medical Units. Recruitment commenced on 19 February 2024 and is anticipated to run for approximately 2 years. This is a three-group dose-finding trial, in which participants are assigned in a 1:1:1 ratio to either Standard NAC (300 mg/kg) or higher doses of 450 mg/kg (Group 1) and 600 mg/kg (Group 2). The primary outcome is the proportion of paracetamol metabolites in the circulation that are directly produced by GSH/NAC detoxification of NAPQI. A higher proportion of these metabolites will indicate that the additional NAC is reducing the amount of toxic paracetamol metabolites in the body. The study will first test the primary outcome on the HiSNAP Group 2 against Standard NAC; only if that is significant will HiSNAP Group 1 be tested against Standard NAC.

Ethics and dissemination

The HiSNAP trial has been approved by the East Midlands (Derby) Research Ethics Committee (reference 23/EM/0129), NHS Lothian Research and Development Department, and the MHRA. Results will be disseminated by peer-reviewed publication, conferences and linked on isrctn.com.

Trial registration number

ISRCTN17516192.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• The study will systematically collect data to assess the safety and efficacy of higher acetylcysteine doses and the current standard dose.

• The study primary end point is measurable in every patient, in contrast to paracetamol overdose trials examining liver injury (which only occurs in approximately 10% of patients).

• The study has pragmatic inclusion criteria—the decision that the patient requires acetylcysteine lies with the clinical team.

• The study seeks to minimise imbalances between treatment groups by stratifying randomisation according to whether a patient is at high risk of liver injury after overdose.

• The study is limited by the primary outcome being biomarker-based. Any benefits on clinical outcomes will still require confirmation.

Introduction

Background

Paracetamol (acetaminophen (APAP)) overdose is a common medical emergency. Across the UK, approximately 100 000 people attend hospital following a paracetamol (APAP) overdose every year (one every 5 min, same as myocardial infarction) and around half need admission for treatment with the antidote acetylcysteine (N-acetylcysteine (NAC)).^{1 2} Around 10% of these patients develop liver injury, and approximately three people die from paracetamol-induced liver failure every week.³ Paracetamol overdose is more common in Scotland than in any other UK region, or indeed globally.

Rationale for the study

There is a lack of robust evidence to guide treatment decisions. The Cochrane Library published a systematic review of interventions for paracetamol overdose in 2018.⁴ The conclusion begins by saying, “These results highlight the paucity of randomised clinical trials comparing different interventions for paracetamol overdose and their routes of administration and the low or very low level quality of the evidence that is available.” This situation has resulted in differences in treatment protocols that are not evidence-based and do not have adequate safety assessments. High-dose NAC and repurposed, expensive drugs are being recommended for off-label use, yet no randomised controlled trials (RCTs) have been performed for these treatment recommendations.⁵ Novel therapeutics are now being developed for APAP overdose, and there is an expectation they will be subject to robust clinical trials prior to clinical use.^{6 7}

Paracetamol is converted in the liver to the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI; figure 1). Glutathione (GSH) is the endogenous antioxidant that protects cells from NAPQI-induced injury. In overdose, GSH stores are depleted, leaving NAPQI free to produce liver damage. NAC prevents paracetamol toxicity by replenishing liver GSH. This protective effect of NAC produces specific metabolites in the circulation, as shown in figure 1. Currently, patients in the UK receive a dose of NAC that is based only on their weight. It is clear from the basic pharmacology, mathematical modelling and observational studies that some patients (particularly those taking a large overdose) may not be receiving enough NAC.8,11

Figure 1. Acetaminophen (APAP, paracetamol) metabolites. The study team has validated assays for APAP, APAP-Glu, APAP-Sul, APAP-GSH, APAP-Mer and APAP-Cys.¹⁷ NAPQI, N-acetyl-p-benzoquinone imine.

In the 1970s, a 20.25-hour regimen for delivering NAC was developed in Edinburgh and elsewhere that became the worldwide standard of care.¹² The dose administered to patients using this regimen is limited by side effects that are directly related to the high initial rate of the NAC intravenous infusion. We designed, trialled and implemented a new regimen for NAC (the ‘SNAP’ regimen) that delivers the same total dose as the 21-hour regimen (300 mg/kg) but with fewer dose-limiting side effects while being quicker to complete (12 hours instead of 21 hours).13,15 This reduction in side effects means we can now safely test the effectiveness of higher doses. The SNAP regimen is endorsed by the UK National Poisons Information Service (NPIS) and the UK Royal College of Emergency Medicine and is now standard care across the UK and is used worldwide.¹⁶ Some patients treated with the SNAP regimen require an additional infusion after 12 hours and receive a higher dose of 500 mg/kg over 22 hours. It is therefore timely to investigate the effectiveness of higher doses of NAC as current evidence is inconclusive.¹⁵

Objectives

In this trial, we will test the hypothesis that a higher NAC dose will increase detoxification of paracetamol. There are observational studies that suggest higher doses of NAC have the potential to reduce liver injury in patients taking large overdoses, but no RCTs have been performed and safety has not been rigorously assessed.¹⁷

Trial design

To assess the risk/benefit profile of high-dose NAC, we propose a three-group dose-finding trial as shown in figure 2. The maximum dose to be studied is 600 mg/kg, chosen because mathematical models predict this will be a sufficient dose to effectively treat 98% of patients.⁹ The other treatment groups will receive 450 mg/kg (1.5× standard care) or 300 mg/kg (standard care). The primary outcome is the proportion of paracetamol metabolites in the circulation that are directly produced by GSH/NAC detoxification of NAPQI (ie, serum %GSH metabolite concentration after 12 hours of NAC). A higher proportion of these metabolites will indicate that the additional NAC is reducing the amount of toxic paracetamol metabolites in the body. For secondary end points, standard and novel liver injury markers will be measured to explore the effect of higher NAC doses on preventing liver injury. Safety will be assessed by systematically recording adverse events (AEs).

Figure 2. Trial design flow chart. Participants are randomised in a 1:1:1 ratio, with the potential to extend the sample size by up to 20% (ie, maximum of 108 patients) if agreed by the Trial Steering Committee, sponsor and funder. NAC, N-acetylcysteine.

Methods and analysis

Study setting

This is a multicentre trial, taking place across several hospitals in Scotland, UK, within Emergency Departments and Acute Medical Units. Recruitment commenced on 19 February 2024 and is anticipated to run for approximately 2 years. Eligible patients who have provided written informed consent will receive 12 hours of study intervention and will be followed up for 7 days. Refer to figure 3 for a study visit flow diagram.

Figure 3. Study visit flow diagram. AMU, Acute Medical Units; ED, Emergency Departments; NAC, N-acetylcysteine.

Patient and public involvement

A patient and public involvement group for Emergency Medicine Research (Sheffield, UK) reviewed and gave feedback on the study design which was actioned prior to grant application submission. In addition, a survey of patients presenting with acute overdose was performed to explore willingness to participate in clinical trials, which demonstrated no difference between this group and general medical patients.

Eligibility criteria

Inclusion criteria

All of

1. Paracetamol overdose presenting to hospital within 24 hours of taking their last dose of paracetamol. All patterns of overdose (single, staggered overdoses and therapeutic excess); accidental or deliberate; paracetamol alone or mixtures of tablets are eligible.

2. Patient deemed to need NAC treatment by the clinical team.

3. Blood paracetamol concentration and alanine aminotransferase (ALT) results are available (clinical care bloods).

4. Provision of informed consent.

5. Adult (16 years old or above).

Exclusion criteria

Any of

1. Patients who do not have the capacity to consent.

2. Patients who are pregnant or breastfeeding.

3. Patients who have previously participated in the study.

4. Patients who, in the opinion of the responsible clinician/nurse, are unlikely to complete the full course of NAC, for example, expressing a wish to self-discharge.

5. Patients detained under the Mental Health Act.

6. Patients already started on NAC treatment.

7. Patients with known viral hepatitis or HIV.

8. Prisoners.

Interventions

The product under investigation is NAC 200 mg/mL concentrate for solution for infusion, delivered by peripheral venous infusion per the SNAP regimen for the treatment of paracetamol poisoning. There will be three dosing groups:

1. Standard NAC as per the SNAP regimen (300 mg/kg): initial loading dose (100 mg/kg in 200 mL) given intravenously over 2 hours, followed by a second dose (200 mg/kg in 1000 mL) infused over 10 hours; or

2. HiSNAP Group 1 (450 mg/kg): initial loading dose (150 mg/kg in 200 mL) given intravenously over 2 hours, followed by a second dose (300 mg/kg in 1000 mL) infused over 10 hours; or

3. HiSNAP Group 2 (600 mg/kg): initial loading dose (200 mg/kg in 200 mL) given intravenously over 2 hours, followed by a second dose (400 mg/kg in 1000 mL) infused over 10 hours.

We will use the SNAP protocol for delivering NAC rather than the 21-hour protocol (as in the NAC marketing authorisation) because the SNAP regimen is the UK standard of care endorsed by the Royal College of Emergency Medicine and the NPIS.¹⁶ All hospital regions in Scotland use the SNAP regimen to deliver NAC treatment. The only variation to standard practice is that NAC doses will be calculated for each patient using their body weight to the nearest kilogram rather than a 10 kg weight-band as is currently used in clinical practice when patients receive Standard NAC (300 mg/kg). There is no placebo arm.

An H1 antihistamine (chlorphenamine 10 mg intravenous) and nebulised salbutamol may be administered to treat anaphylactoid reactions to NAC. Anti-emetic medicines may be given to treat nausea secondary to NAC. Any medication (prescription as well as over-the-counter drugs) or therapeutic intervention deemed necessary for the patient, and which, in the opinion of the investigator, do not interfere with the safety and efficacy evaluations, may be continued.

Outcomes

Primary outcome

The primary outcome measure of efficacy is the proportion of paracetamol metabolites in the circulation that are directly produced by GSH/NAC detoxification of NAPQI (ie, (APAPCys+APAPMer+APAPGSH)/(APAPCys+APAPMer+APAPGSH+APAPGlu+APAPSul+APAP). A higher proportion of these metabolites will indicate that the additional NAC is reducing the amount of toxic paracetamol metabolites in the body.

Secondary outcomes

A full paracetamol metabolite panel (APAP-Cys, APAP-Glu, APAP-Sul, APAP-GSH, APAP-Mer) in serum and urine measured at baseline for serum and urine, 12 hours after the start of NAC for serum, and in the total urine collected in the 12-hour trial period will be conducted. Standard (ALT) and novel liver injury markers (keratin-18, miR-122) will be measured to explore the effect of higher NAC doses on preventing liver damage.¹⁸ International normalised ratio will be measured as a marker of liver synthetic function.

Repeat hospital admission within 7 days, acute liver failure, transfer to the Scottish Liver Transplantation Unit, liver transplantation and death will be reported.

To determine whether there are more AEs with higher NAC doses, adverse reactions will be measured by a Likert scale on the following nine symptoms: nausea, feeling flushed, itchy skin, skin rash, chest pain, headache, feeling breathless, feeling wheezy and tongue/lip swelling. A free-text box is also available. This self-assessment questionnaire will be completed as a baseline and again 12 hours after starting NAC. The number of events and severity of reactions will be used to compare each dosing group.

Safety will be assessed by systematically recording AEs (see ‘Safety assessments’). Anaphylactoid reactions and cerebral oedema are adverse events of special interest (AESI). The symptoms questionnaire will be reviewed as soon as possible after completion and within 7 days of infusion if the questionnaire is available. Any clinically significant events identified by an appropriately delegated physician will be recorded/reported as AEs/serious AEs (SAEs). A clinically significant event is defined as one which requires treatment and/or is recorded in the medical records.

Recruitment

Identification

Patient screening will be undertaken by the nurses of the Emergency Medicine Research Group of Edinburgh. Participant eligibility will be verified by a delegated clinical trial physician after written informed consent has been obtained. Confirmation of eligibility will be recorded within the participants’ medical records and the participant added to the screening log.

The need for NAC treatment will be determined by the blood paracetamol concentration and liver function tests (including ALT) which are measured as part of routine care. Results from routine blood samples taken between 4 hours and 24 hours after the last ingestion of paracetamol overdose may be used for this eligibility assessment.

Pregnancy testing will be carried out prior to randomisation if a potential participant is of childbearing potential and could be pregnant, and this will be recorded in the patient’s medical notes and the electronic case report form (eCRF). Childbearing potential is defined as fertile, following menarche and until becoming postmenopausal unless permanently sterile. Permanent sterilisation methods include hysterectomy, bilateral salpingectomy and bilateral oophorectomy. A postmenopausal state is defined as no menses without an alternative medical cause for 12 months for women over 50 years old and for 24 months for women under 50 years old. A high follicle-stimulating hormone (FSH) level in the postmenopausal range may be used to confirm a postmenopausal state in women not using hormonal contraception or hormonal replacement therapy. However, in the absence of 12 months of amenorrhoea, a single FSH measurement is insufficient.

Randomisation

Randomisation will be carried out using a web-based randomisation service (managed by the Edinburgh Clinical Trials Unit (ECTU)). Participants will be randomised by any appropriately delegated member of the trial team to Standard NAC (300 mg/kg), HiSNAP Group 1 (450 mg/kg) or HiSNAP Group 2 (600 mg/kg) in a 1:1:1 ratio and minimised according to whether a patient meets the criteria of being at high risk of liver injury after overdose. High risk is defined as a hospital presentation ALT activity greater than the upper limit of normal (defined as >50 U/L) or a blood paracetamol concentration over the 300 line on the nomogram (applies only to single-acute overdoses; figure 4). A random element will be included in the minimisation to maintain unpredictability.

Figure 4. Nomogram used in UK clinical practice to determine if treatment is required for paracetamol overdose (100 line) and line used for trial purposes to determine if single-acute overdoses are over the 300 line.

Once a patient is randomised, they will remain in the study and have all outcomes recorded regardless of compliance with study allocation, unless they specifically withdraw consent to have data stored.

Blinding

This is not applicable—the study is not blinded. The trial team members responsible for measuring the primary end point by mass spectrometry will be blinded to the treatment group of each sample.

Participant retention

As participation in the study does not extend significantly beyond the normal duration of treatment and 7-day follow-up is conducted using medical records, we do not anticipate significant issues with participant retention.

Study assessments

Use of study data

Data collection

Data will be collected from consent until 7 days after randomisation. Data items to be collected and data collection points are described in table 1. The local research team as delegated will be responsible for data collection and recording this in the eCRF. Data will be collected by trained and delegated members of the research team.

Table 1. Study assessments.

	Screening	Baseline	Pretreatment	Treatment	Post-treatment	Follow-up
	Day 1	Day 1	Day 1(30 min before NAC)	Day 1(time 0)	Day 1 or 2(conclusion of 12-hour treatment)	Day 7
Screening	X
Consent	X
Eligibility	X
Urine pregnancy test	X
Routine clinical bloods	X				X*
Randomisation	X†X†
Medical history		X
Symptoms questionnaire (AEs)			X‡		X§
AEs identified by research team		XX
Study specific blood			X¶		X**
Study specific urine			X¶		X**
12-hour urine collection (volume)				XX**
NAC treatment per allocation				X
Collection of outcome data (medical records)						X††

^*Routine bloods (±1 hour) routine clinical bloods—if bloods (or individual parameters) are not requested by the clinical team, this will not be recorded as a deviation. If a routine sample is not available, the research team may take an additional sample to measure the end point parameters.

^†Randomisation may take place at any time after consent but before NAC treatment depending on the availability of blood results.

^‡Baseline symptoms questionnaire 0–30 min before NAC treatment begins.

^§5.12-hour symptoms questionnaire (±2 hours). Should ideally be collected as soon as possible after NAC treatment is completed. If participant is discharged before questionnaire us completed, the research team will make no more than three attempts to phone the patient and request the information retrospectively.

^¶Pretreatment interventions 30 min (±15 min).

^**12-hour study-specific samples (±2 hours). Should ideally be collected as soon as possible after 12-hour NAC treatment is completed. If participant refuses blood or urine sample collection, this will not be recorded as a deviation but will be captured in eCRF. Urine: the participant will be asked to empty bladder at 12-hour time point, so final volume can be recorded on eCRF and research sample collected. Blood: where a study-specific blood sample cannot be obtained, the research team will attempt to collect a sample from surplus clinical blood.

^††7-day follow-up (+ 7 days). As day 7 follow-up is collected from the medical records; it can be reviewed and recorded in the eCRF up to 7 days after the time point, so it captures all relevant information.

AE, adverse events; eCRFelectronic case report formNAC, N-acetylcysteine

Data management

Data collected about the participants will be entered into a password-protected, secure database (REDCap; http://www.project-redcap.org) provided and maintained by ECTU.^{19 20} Trained and delegated members of the study team will be given password-protected logins to the database to complete data entry. No directly identifiable information will be entered into the electronic trial database.

Authorised personnel will be given protected access to relevant data. Consent is given from the participant. Examples of this include the Study Statistician in order to produce progress reports to the independent Data Monitoring Committee (DMC) and/or the interim or final statistical analyses; and the Trial Manager to produce blinded logistics reports; the Data Management Team to check the quality of the data and the QA manager for audit and ongoing quality assurance reports; as well as representatives of the sponsor for purposes of audit and monitoring or the regulatory authorities.

Data monitoring

The DMC will meet at least once each year or once 10 patients have been recruited into each trial arm, and subsequently when 20 patients have been recruited to each arm. The committee is an independent, multidisciplinary group with experience in the management of patients with paracetamol overdose, anticipated AEs and the conduct and monitoring of randomised clinical trials. Additional urgent meetings may be arranged if any cases of cerebral oedema occur.

Sample size and analysis plan

We have profiled paracetamol metabolites using samples from the SNAP and MAPP trials (%GSH mean 2.16% (SD 1.81)).¹⁷ External validity is provided by a recent Australian cohort study that reported similar mean and SD for these metabolites.²¹ To detect a 45% difference in this toxicokinetic marker between standard and each of the HiSNAP groups (cf 50% effect size estimated from mouse pilot data), 30 patients per group will give a power of 88% (log-transformation of data, 5% two-sided significance level, analysis by analysis of covariance adjusting for baseline value, assuming correlation 0.62 between baseline and post-NAC %GSH). To control the overall type I error at 5%, we will incorporate a hierarchical hypothesis testing structure. We will first test the primary outcome on the HiSNAP Group 2 against Standard NAC; only if that is significant will we test HiSNAP Group 1 against Standard NAC. Patients with liver injury (and by definition, excessive NAPQI production) have an increase in metabolites of between double to fourfold (200–400%). Therefore, our study is powered to detect a clinically relevant effect size (45%).

The log-transformed primary outcome, %GSH metabolites, will be analysed using a normal linear model, adjusting for the pre-NAC %GSH value and for the continuous form of the baseline variables ALT and blood paracetamol concentration on which the minimisation factor is based. The treatment effect for each HiSNAP group versus Standard NAC will be estimated using the least-squares mean difference and its 95% CI, back-transformed to the original scale to give the ratio for HiSNAP:Standard NAC.

Continuous secondary outcomes will be analysed in the same way as the primary outcome. Categorical secondary outcomes will be analysed using logistic regression, adjusting for the minimisation variables in the same way as in the normal linear model and reporting the treatment effect in each HiSNAP group versus Standard NAC by OR (95% CI). A detailed statistical analysis plan will be finalised prior to database lock and unblinding of the trial statistician.

Exploratory predefined subgroup analyses will investigate the relationship between the treatment effect of each HiSNAP group and baseline levels of ALT and blood paracetamol level. Patients with a blood paracetamol concentration over 300 line on the nomogram are at a high risk of liver injury even with NAC treatment.¹⁰ This will be done separately for each baseline variable by including an interaction term with treatment group in the normal linear model for the primary outcome.

Safety assessments

Participants will be asked about the occurrence of AEs/SAEs at every visit during the study. Open-ended and non-leading verbal questioning of the participant will be used to enquire about AE/SAE occurrence. Participants will also be asked if they have been admitted to hospital, had any accidents, used any new medicines or changed concomitant medication regimens. If there is any doubt as to whether a clinical observation is an AE, the event will be recorded. AEs and SAEs may also be identified via information from support departments, for example, laboratories or participant questionnaires.

The symptoms questionnaire will be reviewed as soon as possible after completion and within 7 days if the questionnaire is available and any clinically significant events identified by the principal investigator (PI) (or delegated physician) will be recorded/reported as AEs/SAEs. A clinically significant event is defined as one which requires treatment and/or is recorded in the medical records.

Anaphylactoid reactions and cerebral oedema are AESI and will be recorded and reported. The DMC will review the occurrence of AESI and their related symptoms by treatment group at each meeting. Any cases of confirmed cerebral oedema confirmed by the PI will immediately halt the trial and mandate DMC review before any further participants are randomised as this is a stopping criterion.

Monitoring and oversight

The HiSNAP trial is an investigator-led study, sponsored by the Academic and Clinical Central Office for Research and Development for NHS Lothian/University of Edinburgh (ACCORD, reference AC23035, ACCORD@nhslothian.scot.nhs.uk). Trial oversight is directly provided by an independent Trial Steering Committee and a separate independent DMC who will provide advice and review of safety.

Study amendments and current status

The study protocol currently in effect is V.6, 3 July 2024. A protocol version history is provided in online supplemental 1. Recruitment commenced 20 March 2024. At the time of authorship, 16 patients have been recruited and the trial is expected to complete recruitment on schedule.

Ethics and dissemination

The trial will be conducted according to the ethical principles of the Declaration of Helsinki 2013 and was approved by the East Midlands (Derby) Research Ethics Committee, 26 September 2023 (reference 23/EM/0129), NHS Lothian Research and Development Department and the MHRA. Good Clinical Practice regulations will be followed, and written informed consent will be obtained from all participants. A copy of the consent form is provided as online supplemental material.

No trial results will be shared while the trial is ongoing. When the trial completes, data will be shared via presentation, publication and may be available on request from the trial chief investigator, decided on a case-by-case basis. Data sharing may be restricted in line with patent or commercial requirements for as short a time as possible.

Results will be disseminated by peer-reviewed publication, conferences and linked on isrctn.com. Publication content will follow the Reporting of surrogate end points in randomised controlled trial reports (CONSORT-Surrogate) Extension of the CONSORT 2010 statement.²² Ownership of data arising from this study resides with the study team at the Sponsor Organisation. The study team will follow the International Committee of Medical Journal Editors guidelines on authorship. Requests for data access should be sent to the corresponding author.