Administration of Ondansetron With P-glycoprotein Inhibitor Tariquidar in Patients With Neuropathic Pain
Phase: PHASE1
Conditions: Neuropathic Pain
Sponsor: Washington University School of Medicine

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RESEARCH PROTOCOL 
 
 
Administration of ondansetron with P-glycoprotein inhibitor tariquidar in patients with neuropathic pain 
  
 
Principal Investigator: 
Simon Haroutounian, PhD 
Associate Professor of Anesthesiology 
Chief, Division of Clinical and Translational Research 
Department of Anesthesiology, Washington University School of Medicine 
660 S. Euclid Ave, Campus Box 8054,  
Saint Louis, MO, 63110 
E-mail: simon.haroutounian@wustl.edu 
 
Co-investigators: 
 
 
Robert A. Swarm, MD 
Professor of Anesthesiology 
Chief, Division of Pain Management 
Department of Anesthesiology, Washington University School of Medicine 
E-mail: swarmr@wustl.edu  
 
Katharine N. Gurba, MD, PhD 
Department of Anesthesiology, and Pain Management Center,  
Washington University School of Medicine 
E-mail:gurbak@wustl.edu 
 
Lara Crock, MD, PhD 
Department of Anesthesiology, and Pain Management Center,  
Washington University School of Medicine 
E-mail: crockl@wustl.edu  
 
Yan Yan, MD, PhD 
Professor of Clinical Epidemiology and Biostatistics; Department of Surgery 
Washington University School of Medicine 
yany@wustl.edu  
 
 
Version Date:  September 8, 2020 
Revised:  October 29, 2020 
          April 21,, 2022 
 
 
 
 
 
Short Title: Ondansetron-Tariquidar combination in patients with neuropathic pain 
 
 
 

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1.  SYNOPSIS 
 
Study Title 
Administration of ondansetron with P-glycoprotein inhibitor tariquidar in 
patients with neuropathic pain 
Objective 
To determine the pharmacokinetics and tolerability of co-administration of 5-
HT3R antagonist ondansetron with a P-glycoprotein inhibitor tariquidar, in 
patients with neuropathic pain. 
Hypothesis 
Co-administration with tariquidar will increase the cerebrospinal fluid (CSF) to 
plasma ratio of ondansetron after intravenous administration.  
 
Co-administration of a single dose 16mg ondansetron with 4mg/kg tariquidar 
will be tolerable in patients with neuropathic pain 
Study Period 
Planned enrollment duration: Approximately 24 months 
Planned study duration: Approximately 30 months 
Number of Subjects 
28 evaluable patients with peripheral neuropathic pain  
Study Drug  
Two infusions of 16mg intravenous ondansetron co-administered with 
intravenous 4mg/kg tariquidar or placebo (D5W), 3 weeks apart.  
If intolerable, the dose of ondansetron will be reduced to 12mg or 8mg in a 
stepwise fashion (see section 3.2). 
Study Design 
Prospective, randomized, double-blind, placebo controlled, cross-over proof 
of concept study. 
Inclusion and 
Exclusion Criteria 
Inclusion criteria: 
1. Age 18-70;  
2. Documented diagnosis of neuropathic pain due to damage or disease 
affecting the peripheral nervous system; 
3. At least Probable neuropathic pain grading1; 
4. Pain duration >3 months; 
5. Average pain intensity ≥4 on 0-10 numerical rating scale (NRS). 
Exclusion criteria: 
1. Current pregnancy or lactation; 
2. Moderate-severe kidney or liver dysfunction; 
3. Active cardiac arrhythmias (non-sinus rhythm), Long QT syndrome, or 
QTc interval >450msec; 
4. Congestive heart failure 
5. Abnormal troponin values at screening visit; 
6. Current treatment with MAO inhibitors, mirtazapine, SSRI or SNRI 
antidepressants, except duloxetine up to 60mg/day, escitalopram up 
to 10mg/day, or citalopram up to 20mg/day; 
7. Current treatment with tapentadol, tramadol, or fentanyl; 
8. Current treatment with P-glycoprotein substrate drugs with narrow 
therapeutic window, e.g. digoxin; 
9. Current treatment with tricyclic antidepressant medications (e.g. 
amitriptyline, desipramine, imipramine) at a dose >25mg/day; 
10. Ongoing use of any of the following medications with known effects 
on Pgp function: carbamazepine, phenytoin, phenobarbital, 
cyclosporine, clarithromycin, erythromycin, ritonavir, verapamil, 
rifampicin, St. John’s wort; 
11. Current treatment with QT-prolonging drugs, and drugs known to 
have a significant interaction with ondansetron or other P-
glycoprotein substrates (see section 2.3.3.); 

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Measurements 
1. Venous blood sampling for pharmacokinetics: At 0, 15, 30, 60, 90, 120, 
180, and 240 minutes from the beginning of ondansetron infusion. 
2. A CSF sample will be obtained via lumbar puncture between 30 and 180 
minutes from the beginning of ondansetron infusion, for the 
assessment of Ondansetron concentration in the CSF using population 
PK approach. 
3. Vital signs (pulse oximetry, blood pressure): before infusion and 30, 60, 
120, 180 and 240 minutes after end of ondansetron infusion.  
4. 12-lead ECG: At baseline and prior to discharge.  
5. Troponin: prior to discharge.  
6. Intensity of spontaneous pain (on 0-10 NRS): At baseline, and after 
ondansetron administration; every 10 minutes for 60 minutes, every 15 
minutes thereafter for a total of 120 minutes, every 30 minutes until 
240 minutes, hourly on the study day until bedtime, and daily for 3 
weeks thereafter.  
7. Neuropathic Pain Symptom Inventory (NPSI) pain descriptors: Prior to 
and 70 minutes after ondansetron administration. 
Statistical 
Methodology 
Primary Outcome:  
The primary objectives are to evaluate the pharmacokinetics and the tolerability 
of ondansetron and tariquidar single dose iv co-administration (in comparison 
to ondansetron administration alone) in patients with neuropathic pain. 
 
Secondary outcomes: 
1. Change in spontaneous pain intensity (measured on 0-10 NRS) from 
baseline to 60-120 minutes after ondansetron IV infusion, compared 
between two sessions with and without tariquidar. The pain severity 
values obtained at 60, 90, and 120 minutes will be averaged to derive 
the outcome measure. The values will be compared via paired t-test (or 
Wilcoxon signed-rank test, if data not normally distributed). 
2. Changes in NPSI total score and sub-scores (burning pain, paroxysmal 
pain, paresthesia/dysesthesia score) will be compared between 
treatment sessions;  
3. The association between baseline Conditioned Pain Modulation (CPM) 
magnitude (ΔCPM) and the % pain reduction from baseline will be 
determined by bivariate regression. A significant association between 
the two measures will be indicative of differential analgesic response as 
a function of descending pain modulation;  
4. Exploratory analyses will be performed to determine if variables such as 
sex, weight, baseline wind-up ratio (WUR), peak ondansetron plasma 
concentration, and P-glycoprotein genotype affect the difference in 
pain reduction between the two study sessions. 

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2.  STUDY PROTOCOL 
 
2.1 Background and Significance 
Neuropathic pain (NeuP) affects more than 16 million Americans2 and results in considerable impairment in 
quality of life.3 Five of every six patients with NeuP do not achieve adequate pain relief with currently approved 
medications;4 moreover, efficacious drugs such as opioids and tricyclic antidepressants have major safety concerns. 
Therefore, new therapeutic approaches are urgently needed to reduce the burden of NeuP. 
Serotonergic 5-HT3 receptors (5-HT3R) in the central nervous system (CNS) have been identified as a promising 
pharmacological target for neuropathic pain.5,6 Normally, descending modulation from the brainstem inhibits the 
sensitization of spinal cord dorsal horn neurons through serotonin activity.7 However, after peripheral nerve 
damage, neurons in the dorsal horn demonstrate overexpression of 5-HT3R, the only (excitatory) ion channels 
among serotonin receptors.6,8 As a result, the character of descending serotonergic modulation in NeuP can shift 
from inhibitory to excitatory (facilitatory).9,10 This finding is supported by studies demonstrating that local 
intrathecal delivery of 5-HT3R antagonists such as ondansetron alleviates mechanical and thermal hypersensitivity 
in animal models of nerve injury.5,11-13 In contrast, these analgesic effects are not consistently observed with 
systemic administration of these drugs in either preclinical pain models14,15 or in analgesic clinical trials.16,17 This 
discrepancy can be explained by the fact that currently available 5-HT3R antagonists are substrates of efflux 
transporters such as the P-glycoprotein (Pgp), located at the capillaries of blood-brain barrier (BBB) and blood-spinal 
cord barrier (BSCB).18,19 Our preliminary data and others20 suggest that this efflux limits the CNS exposure to 
systemic 5-HT3R antagonists, preventing neuropathic pain relief. Bypassing Pgp-mediated efflux is expected to 
increase the concentrations of these drugs at their CNS sites of action and thus provide adequate relief in 
neuropathic pain. 
In addition to CNS penetration, the choice of appropriate pain condition and patient somatosensory 
characteristics are important for preventing a “translational mismatch” between preclinical and clinical study 
design.21 While 5-HT3R overexpression in rodent nerve injury models contributes to the descending facilitation,11,22 
none of the previous clinical studies with 5-HT3R antagonists have attempted to select patients who would exhibit 
a phenotype suggesting descending facilitation, where the intervention is expected to work. Moreover, in one study 
that undertook the effort of quantifying descending modulation, most patients exhibited efficient inhibition (and 
not facilitation).23 Interestingly, tropisetron was efficacious in fibromyalgia,24,25 a condition in which most patients 
tend to exhibit descending facilitation.26,27 Determining descending pain modulation phenotype is promising for 
predicting responses to certain analgesics,28,29 and recent recommendations for such sensory phenotyping in 
chronic pain studies have been published.30  
 
 
2.2 Preliminary Data 
Our preliminary data on conditioned pain modulation (CPM), a 
surrogate measure of descending inhibition vs. facilitation (Fig. 1) 
demonstrate that while in healthy adults less than 20% exhibit descending 
facilitation, 40-50% of patients with peripheral NeuP exhibit various 
degrees of descending facilitation. We expect to have sufficient balance 
between descending inhibition and facilitation among enrolled NeuP 
patients, to determine the association between this phenotype and pain 
reduction. In a recent pharmacokinetic study of ondansetron disposition 
in the CSF, a 7-fold difference was observed between plasma and CSF 
concentrations of ondansetron after a single 16-mg infusion.31 
 
We have also performed a series of experiments in rats to determine the 
distribution properties of ondansetron (in plasma, heart, and brain) in the 
setting of P-glycoprotein (Pgp) genetic knockout (KO) or Pgp inhibition 
with tariquidar (Figure 2). Plasma concentrations (2A) of ondansetron 
were not different between wild type (WT) rats, Pgp KO rats, or after pre-administration of 15mg/kg tariquidar in 
Fig 1. Conditioned pain modulation 
(CPM) in NeuP and healthy controls.  
DPN- diabetic peripheral neuropathy; CIPN- 
chemotherapy-induced peripheral neuropathy;  

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wild-type rats. Heart concentrations (2B) of ondansetron in Pgp KO rats compared with WT rats were not different 
at 30 and 60-min time points, and were lower at 15- and 120-min time points. In rats pre-treated with tariquidar 
(only 1-h time point available), the heart concentrations of ondansetron were not different from wild-type or Pgp 
knockout.  Brain concentrations (2C) of ondansetron were higher in Pgp KO rats and in tariquidar-treated rats, 
compared with ondansetron administered to WT rats. There was no difference in brain concentrations between 
Pgp KO rats and rats pre-treated with tariquidar. 
 
 
 
 
 
 
0.01
0.1
1
10
0
1
2
3
4
5
Ondansetron  (ug/mL)
Time (h)
Plasma
Cp WT
Cp Pgp KO
Cp - OND only
Cp - OND+TQR
A 
0.01
0.1
1
10
0
1
2
3
4
Ondansetron (ug/mL)
Time (h)
Heart
Chrt WT
Chrt Pgp KO
Chrt - OND only
Chrt - OND+TQR
B 
0.01
0.1
1
10
100
0
1
2
3
Ondansetron (ug/mL)
Time (h)
Brain
Cbr WT
Cbr Pgp KO
Cbr - OND only
Cbr - OND+TQR
C 
Figure 2.  
Tissue concentrations of 
ondansetron 
in 
rats 
(mcg/mL 
tissue). 
Male 
Sprague-Dawley rats were 
injected with 10mg/kg IV 
ondansetron (OND) via a 
jugular vein cannula and 
sacrificed at indicated time 
points. 
Blue 
diamonds: 
concentrations 
of 
ondansetron in wild-type 
(WT) rats. Red squares: 
concentrations 
of 
ondansetron in P-gp knock-
out 
(KO) 
rats. 
Orange 
diamond: single time-point 
(1h) 
concentration 
of 
ondansetron. Green square: 
single 
time-point 
(1h) 
concentration 
of 
ondansetron; 
tariquidar 
(TQR) was administered 30 
min prior to ondansetron 
via a jugular vein cannula.  

4/21/2022 
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In addition, a series of experiments were performed to determine the concentration of ondansetron in rat CSF, 
spinal cord, and brain, performed in wild-type rats, with and without intravenous 7.5mg/kg tariquidar co-
administration.  
 
Plasma to CSF partition coefficient (Cp) of ondansetron, after 10mg/kg dose, in wild-type male rats 
without tariquidar was 0.27±0.3; plasma to CSF partition coefficient (Cp) of ondansetron, with co-
administration of 7.5mg/kg tariquidar was 0.36±0.16 
 
 
Plasma to spinal cord partition coefficient (Cp) of ondansetron, after 10mg/kg dose, in wild-type male rats 
without tariquidar was 1.11±0.4; plasma to spinal cord partition coefficient (Cp) of ondansetron with co-
administration of 7.5mg/kg tariquidar was 4.72±2.8 
 
 
Plasma to brain partition coefficient (Cp) of ondansetron, after 10mg/kg dose, in wild-type male rats 
without tariquidar was 1.75±1.2; plasma to brain partition coefficient (Cp) of ondansetron with co-
administration of 7.5mg/kg tariquidar was 5.88±5.6 
In summary, co-administration of tariquidar resulted in 1.3 fold increase in CSF to blood partitioning coefficient, 
while in parallel, spinal cord to blood partitioning coefficient increased 4.2-fold, and brain to blood partitioning 
coefficient increased by 3.4-fold.  
 
Cardiovascular safety data from the first 8 participants in the human pharmacokinetic study per IND 140361 
(NCT03809234; 2 participants with 4mg ondansetron, 4 participants with 8mg ondansetron, and 2 participants 
with 16 mg ondansetron, with and without 4mg/kg tariquidar) demonstrate the following: 
 
None of the participant had any increase in troponin at any point in the study.  
 
The QTc interval changes (before to after ondansetron) with and without tariquidar were -2.5±16.1 ms vs 
2.9±13.1 ms, respectively (non-significant difference). 
 
Maximum change in systolic blood pressure was an 8.6±9.7 mmHg increase, two hours after infusion of 
ondansetron with placebo, and a 5.7± 8.2 mmHg decrease, 1.5 hours after infusion of ondansetron with 
tariquidar, across doses. 
 
Maximum change in diastolic blood pressure was an 3.4±7.7 mmHg increase, two hours after infusion of 
ondansetron with placebo, and an 7.1±9.6 mmHg increase, 105 minutes after infusion of ondansetron 
with tariquidar, across doses. 
 
Maximum change in heart rate was an 4.2±13.1 BPM increase, 2 hours after infusion of ondansetron with 
placebo, and an 12.1±17.0 BPM increase, 105 minutes after infusion of ondansetron with tariquidar, 
across doses. 
 
 
 
 
 

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The CSF concentrations of ondansetron have been analyzed only in the 8mg dose group. 
 
 
  
 
 
 
Based on rodent data, we expect that the increase in spinal cord and brain exposure with tariquidar co-
administration will be higher than the proportionate CSF exposure. However, we don’t expect as high as 3-4 fold 
higher exposure in humans, as Pgp expression in rat blood-brain barrier is substantially higher than in humans. 
 
The tolerability and pharmacokinetic parameters of ondansetron co-administered with tariquidar among patients 
with neuropathic pain is unknown, and should be evaluated before large-scale efficacy clinical studies. 
 
2.3 Objective 
The primary objective of the study is to determine the pharmacokinetics and tolerability of co-administration of 5-
HT3R antagonist ondansetron with a P-glycoprotein inhibitor tariquidar, in patients with neuropathic pain. 
 
Secondary objectives are to assess the change in pain intensity and neuropathic pain descriptors between the two 
study arms, and to determine psychophysical determinants of analgesic response. 
 
2.3.1 Subject Selection 
Patients with neuropathic pain meeting inclusion criteria will be recruited from the Saint Louis community until 28 
participants have completed both study sessions.  
 
2.3.2 Inclusion Criteria 
1. Age 18-70;  
2. Documented diagnosis of neuropathic pain due to damage or disease affecting the peripheral nervous 
system (e.g. Diabetic neuropathy, Post-herpetic neuralgia, Chemotherapy-induced peripheral neuropathy, 
Lumbar radiculopathy, or Post-traumatic neuropathy); 
3. At least Probable neuropathic pain grading (per Finnerup et al criteria)1;  
4. Pain duration >3 months; 
-8
-4
0
4
8
12
16
20
0
1
2
3
4
OND CSF concentration (ng/mL)
Time (hr)
Ondansetron (OND) CSF concentration, 8mg single IV dose, n=3
OND + TQD
OND + placebo
Figure 3. CSF concentrations of ondansetron 8mg iv infusion (over 15 min) with and without 
concomitant administration of 4mg/kg tariquidar (TQD) in healthy volunteers, (0-4 hours). The 
area under the concentration time curve (AUC0-4) is 14.1 ng*h/mL for ondansetron administered 
with placebo (D5W solution), and 28.0 ng*h/mL for ondansetron administered with tariquidar. 

4/21/2022 
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5. Average pain intensity ≥4 on 0-10 numerical rating scale (NRS). 
 
 
2.3.3 Exclusion Criteria 
Participants will not be enrolled if any of the following criteria exist: 
 
1. Current pregnancy or lactation; 
2. Moderate-severe kidney or liver dysfunction; 
3. Active cardiac arrhythmias (non-sinus rhythm), Long QT syndrome, or QTc interval >450msec; 
4. Congestive heart failure 
5. Abnormal troponin values at screening visit 
6. Current treatment with MAO inhibitors mirtazapine, SSRI or SNRI antidepressants, except duloxetine up 
to 60mg/day, escitalopram up to 10mg/day, or citalopram up to 20mg/day; 
7. Current treatment with tapentadol, tramadol, or fentanyl 
8. Current treatment with P-glycoprotein substrate drugs with narrow therapeutic window, e.g. digoxin. 
9. Current treatment with tricyclic antidepressant medications (e.g. amitriptyline, desipramine, imipramine) 
at a dose >25mg/day  
10. Ongoing use of any of the following medications with known effects on Pgp function: carbamazepine, 
phenytoin, phenobarbital, cyclosporine, clarithromycin, erythromycin, ritonavir, verapamil, rifampicin, St. 
John’s wort; 
11. Current treatment with QT-prolonging drugs, and drugs known to have a significant interaction with 
ondansetron or other P-glycoprotein substrates (see below): 
 
Antiretrovirals of Protease inhibitor (e.g. Ritonavir, Saquinavir) or Non-nucleoside reverse transcriptase 
inhibitors (e.g. Efavirenz, Zidovudine) family. 
 
Phenytoin, Carbamazepine, Oxcarbazepine, Rifampin 
 
Amiodarone 
 
Azole antifungals (e.g. Itraconazole, Fluconazole) 
 
Macrolide antibiotics (Erythromycin, Clarithromycin) 
 
Cimetidine 
 
Lithium 
 
Methylene Blue 
 
Non-DHP calcium channel blockers Verapamil and Diltiazem 
 
First generation antipsychotic medications Thioridazine, Haloperidol, Chlorpromazine, and Pimozide 
 
Second generation antipsychotic medications Ziprasidone and Quetiapine 
 
Antihistamine Terfenadine, 
 
Antidepressants Trazodone and Bupropion  
 
Antiarrhythmics Propafenone, Flecainide, and Procainamide 
 
Fluoroquinolone antibiotics  Norfloxacin, Ofloxacin, and Ciprofloxacin 
 
Cisapride  
 
Other strong inhibitors or inducers of Cytochromes P450 2D6 or 3A4. 
 
Other strong inhibitors or inducers of P-glycoprotein 
 
 
 
 
 
 
 
 
2.4 Design and Procedures 

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2.4.1 Study Design 
 
Randomization and Blinding: The study is designed as a prospective, randomized, double-blind, cross-over 
experiment in patients with neuropathic pain. After signing informed consent and meeting eligibility criteria, 
participants will receive two 16 mg IV infusions of ondansetron, approximately 3 weeks apart. Participants will be 
randomized to the order of tariquidar (4mg/kg dose in 500mL D5W) and placebo (D5W solution) co-administration 
with ondansetron. 
 
2.4.2 Pre-Study Period 
Potential participants will be identified from the Pain Management Center of Washington University in St Louis, via 
ads in the community, Epic-based (and IRB-approved) direct recruitment tools, and through Volunteer for Health 
(VFH) organization. Once the potential subject has contacted a member of the research team, a research 
coordinator will provide a description of the project either by phone, email or mail, at the discretion of the potential 
subject. Potential subjects will be asked to undergo telephone screening by a trained research coordinator to assess 
preliminary eligibility for the study, although this may be done in person at the request of the volunteer. 
 
 
The following screening information will be collected by phone: age, height and weight, known allergies, 
complete medical history, neuropathic pain description and intensity. Name and dosing regimen of any over the 
counter and prescription medications or dietary supplement will be collected as well.  
 
 
Potential subjects meeting all inclusion and exclusion criteria during the telephone screening will be scheduled 
for a screening/consent visit at Washington University/Barnes-Jewish Hospital medical center campus. The 
following will take place at the screening/consent visit, which will occur approximately 1 week before the first 
treatment study session: 
 
 
Verbal discussion of the study procedures, benefits, and potential risks  
 
Subjects reads, understands, and signs informed consent (subjects will also be provided with a written 
copy); 
 
Urine pregnancy test for women of childbearing age; 
 
Brief history and physical, including vital signs, height and weight measurements, pain history and intensity, 
and medications;  
 
Blood draw for troponin, comprehensive metabolic panel (CMP) and complete blood count (CBC); if 
subject’s lab tests results are available within the past 30 days, we will utilize these. 
 
12-lead ECG test to rule out QTc interval > 450ms 
 
Completion of the following questionnaires: Brief Pain Inventory (BPI), Hospital Depression and Anxiety 
Scale (HADS), and Pain Catastrophizing Scale (PCS), Neuropathic Pain Symptom Inventory (NPSI), Color-
Word Matching Stroop Test (CWMST); 
 
Completion of standardized pain testing: conditioned pain modulation (CPM), wind-up ratio (WUR) to 
punctate mechanical stimulation, and quantitative sensory testing (QST) for cold, heat, mechanical, and 
vibration sensitivity. 
 
Pain diary to track daily pain levels. 
 
 
Women of childbearing potential with positive urine pregnancy test will be excluded from the study. Women 
of childbearing potential must agree not to try to become pregnant during the study period and 28 days after study 
completion. Women of childbearing potential must also use a highly effective birth control method (such as oral 
contraceptives, condoms, or intrauterine device) throughout and 28 days after the study. Male participants must 
agree to use a highly effective method of contraception from screening to 90 days after last study drug 
administration. 
 
Concomitant analgesic use: 

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Stable doses of analgesics or adjuvant pain medications such as gabapentin, pregabalin, around the clock opioids 
(with a total daily dose not exceeding 50mg oral morphine equivalent), except those listed in the exclusion criteria, 
will be allowed, as long as no dose change has occurred 2 weeks prior to the study. PRN analgesic use (NSAIDs, 
short-acting opioids, acetaminophen) will not be allowed 48 hours prior to study participation. If the participant is 
unable to abstain from short-term analgesics for the 48 hours prior to the study intervention, the participant can 
decide whether to reschedule the intervention, or withdraw from the study. The participants can resume their 
short-acting analgesics 24 hours after the study intervention. The study coordinator will contact the participants by 
phone once a week during the washout period, and for 3 weeks after the second (crossover) intervention, to address 
any questions or concerns the participants may have.  
 
SNRI neuropathic pain medications, tramadol, fentanyl and tapentadol will not be allowed, per exclusion criteria.  
 
2.4.3 Study Period 
Participants will be studied in the Washington University Pain Center, or in the Clinical Research Unit (CRU) 
of Washington University School of Medicine, with continued presence of research and nursing personnel. The 
study will comprise three sessions over an approximately 28-day period (Figure 2A). As previously described, the 
screening visit will take place within about 7 days prior to the first study visit. The screening visit can be coordinated 
on the same morning as test day 1.   
Intervention: 
Each participant will receive two IV infusions of ondansetron, 3 weeks apart, in randomized order. Placebo 
(D5W) or tariquidar (4mg/kg dose in D5W) will be administered IV over 60 minutes according to randomization 
schedule. Ondansetron will be diluted in 100mL D5W, and tariquidar will be diluted in 500mL D5W. Starting 30 
minutes after the initiation of tariquidar/placebo, a 15-min ondansetron co-infusion will be given, to time its peak 
brain concentration with potent Pgp inhibition (Figure 3). A 3 weeks interval between the two sessions (> 5 half-
lives) will allow the elimination of the first dose of ondansetron and tariquidar. A 12-lead ECG will be repeated prior 
to the second infusion to confirm that subjects still have a normal QTc and sinus rhythm. Any subject who has a 
QTc>450ms will not receive the second ondansetron dose and will be withdrawn from the study. A blood sample 
will be drawn at the 240 min time point for troponin levels. If troponin levels are elevated, the patient will be 
immediately evaluated by a physician and a cardiology consult will be obtained. Further assessment and treatment 
of these subjects is described below in section 3, “Management of Intercurrent Events”.  
Vital signs (pulse oximetry and 3-lead ECG; non-invasive blood pressure) will be monitored before infusion and 
approximately 30, 60, 120, 180 and 240 minutes after end of ondansetron infusion.  A 12-lead ECG will be performed 
at baseline and before the participants are discharged.   
Adverse effects will be collected by spontaneous participant reporting, but also asked about every 30 minutes. 
On each of the intervention dates, women of childbearing potential will undergo urine β-hCG testing, and will be 
excluded if pregnancy test is positive. Participants will complete a brief pain assessment, as well as WUR and CPM 
testing at baseline (Figure 2B). Thereafter, two IV catheters will be inserted in participants’ arms. One will be utilized 
for drug infusion and the catheter in the contralateral arm will be used for serial blood sampling. One 5-ml blood 
sample will be obtained for P-glycoprotein polymorphism analysis. At study visit 1 and study visit 2, an additional 5-
ml baseline blood sample will be obtained for future research with participant consent. Participants will report 
baseline spontaneous pain (0-10 NRS) and complete a baseline NPSI. Placebo (D5W solution) or tariquidar (4mg/kg 
dose) will be administered IV over 60 minutes. Starting 30 minutes after the initiation of tariquidar/placebo, a 15-
min ondansetron co-infusion will be given, infused via Y-site, to time its peak brain concentration with Pgp 
inhibition. The intensity of spontaneous pain (on 0-10 NRS) will be assessed every 10 minutes for 60 minutes, and 
every 15 minutes thereafter for a total of 120 minutes, then every 30 min for a total of 240 minutes. WUR and CPM 
testing will be repeated 90 minutes after the end of ondansetron infusion (i.e., 105 minutes after the beginning of 
ondansetron infusion). Participants will record self-reported pain hourly on the study day until bedtime, and daily 
for 3 weeks thereafter. NPSI pain descriptors will be assessed prior to, and 70 minutes after, the beginning of 
ondansetron infusion. Additionally, blood samples for ondansetron concentrations will be obtained at baseline 
(time zero) and approximately at 15, 30, 60, 90, 120, 180, and 240 minutes from the beginning of ondansetron 

4/21/2022 
11 
 
infusion (See Table 1 for full summary of the measurement schedule.) Plasma will be processed and stored in 
aliquots at -80°C until analysis. A 3-mL CSF sample (per participant’s consent) will be obtained via lumbar puncture 
between 30 and 180 minutes from the beginning of ondansetron infusion, for the assessment of ondansetron 
concentration in the CSF using population PK approach as we previously described31. Lumbar puncture will not be 
performed if participant is currently taking anticoagulant treatment or if the participant has a contraindication to 
the lumbar puncture per investigator discretion. 
 
Participants will return for a follow-up visit approximately 3 weeks after the second infusion, to complete adverse 
effects assessment, and blood sampling for blood chemistry and blood count. 
 
 
 
 
 
 
 
 
2.4.4 Minimization of Bias 
There will be no specific ethnic, racial, or gender background for enrollment. The study is designed as cross-over in 
order to minimize inter-subject variability. The participants and investigators will be blinded to treatment allocation 
sequence. 
 
2.4.5 Observations and Measurements 
 
2.4.5.1 Primary Outcome Measures 
Primary outcome:  
The primary objectives are to evaluate the pharmacokinetics and the tolerability of ondansetron and tariquidar single 
dose iv co-administration (in comparison to ondansetron administration alone) in patients with neuropathic pain. 
 
Pharmacokinetics: 
-Venous blood sampling for pharmacokinetics will be obtained: at 0, 15, 30, 60, 90, 120, 180, and 240 minutes from 
the beginning of ondansetron infusion. 
-A CSF sample will be obtained via lumbar puncture between 30 and 180 minutes from the beginning of 
ondansetron infusion, for the assessment of ondansetron concentration in the CSF using population PK approach. 
-CSF to plasma ratio of ondansetron will be calculated, and compared between the two study arms 
 
Tolerability: 
-Spontaneous reporting of adverse effects 
Figure 3. Outline of study design and procedures.  
A. Overall crossover design, including screening and testing visits.  
B. Testing day procedures. NRS=numerical rating scale; NPSI=neuropathic pain symptom inventory; WUR=wind-up 
ratio; CPM=conditioned pain modulation 

4/21/2022 
12 
 
-Vital signs (pulse oximetry, blood pressure): before infusion and 30, 60, 120, 180 and 240 minutes after end of 
ondansetron infusion.  
-12-lead ECG: At baseline and prior to discharge after each treatment session.  
-Troponin T: at screening and prior to discharge after each treatment session.  
-Blood chemistry (CMP) and count (CBC) at baseline and at 3-week time point after each treatment session. 
-In patients who are concomitantly treated with SSRI or SNRI medications that are allowed for inclusion (duloxetine 
≤60mg/day, citalopram ≤20mg/day, or Escitalopram ≤10mg/day, additional assessments will be performed at the 
time of vital sign assessments (baseline and 30, 60, 120, 180 and 240 minutes after end of ondansetron infusion) to 
evaluate confusion, restlessness, dilated pupils, muscle twitching or rigidity, and sweating. This will be performed 
to identify signs suggestive of serotonin syndrome.  
 
2.4.5.2 Secondary Outcome Measures 
Secondary outcome measures include:  
1. Change in spontaneous pain intensity (measured on 0-10 NRS) from baseline to 60-120 minutes after 
ondansetron IV infusion, compared between two sessions with and without tariquidar. The pain severity 
values obtained at 60, 90, and 120 minutes will be averaged to derive the secondary outcome measure. The 
values will be compared via paired t-test (or Wilcoxon signed-rank test, if data not normally distributed). 
2. Changes in NPSI total score and sub-scores (burning pain, paroxysmal pain, paresthesia/dysesthesia score) 
will be compared between treatment sessions;  
3. The association between baseline Conditioned Pain Modulation (CPM) magnitude (ΔCPM) and the % pain 
reduction from baseline will be determined by bivariate regression. A significant association between the 
two measures will be indicative of differential analgesic response as a function of descending pain 
modulation;  
Exploratory analyses will be performed to determine if variables such as sex, weight, baseline wind-up ratio (WUR), 
peak ondansetron plasma concentration, and P-glycoprotein genotype affect the difference in pain reduction 
between the two study sessions. 
A population PK approach will be used to model CSF concentrations of ondansetron with and without tariquidar co-
administration.  
 
Table 1. Study Day Timeline 
 
Drug 
Blood* 
PK 
NRS 
NPSI 
CPM 
WUR 
QST 
Questio
nnaires 
Screening   
TQD 
OND 
x 
Blood 
CSF 
x 
x 
x 
x 
x 
x 
Study day (min after ondansetron infusion start) 
Baseline 
 
 
x 
x 
 
x 
x 
x 
x 
x 
 
-30 min 
TQD / 
placebo 
Infusion 
 
 
 
 
 
 
 
 
 
 
0 min 
OND 
infusion 
 
x 
 
x 
 
 
 
 
 
10 min 
 
 
 
x 
 
 
 
 
 
15 min 
 
x 
 
 
 
 
 
 
 
20 min 
 
 
 
 
x 
 
 
 
 
 
30 min 
 
 
x 
x** 
x 
 
 
 
 
 
40 min 
 
 
 
 
x 
 
 
 
 
 
50 min 
 
 
 
 
x 
 
 
 
 
 
60 min 
 
 
 
x 
x 
 
 
 
 
 
70 min 
 
 
 
 
 
x 
 
 
 
 
75 min 
 
 
 
 
x 
 
 
 
 
 
90 min 
 
 
 
x 
x 
 
 
 
 
 
105 min 
 
 
 
 
x 
 
x 
x 
 
 
120 min 
 
 
 
x 
x 
 
 
 
 
 
135 min 
 
 
 
 
x 
 
 
 
 
 

4/21/2022 
13 
 
150 min 
 
 
 
 
x 
 
 
 
 
 
165 min 
 
 
 
 
x 
 
 
 
 
 
180 min 
 
 
 
x 
x 
 
 
 
 
 
240 min 
 
 
 
x 
 
x 
 
 
 
 
 
Hourly after discharge- study day 
 
 
 
x 
 
 
 
 
 
Post-study day (weeks) 
 
 
 
 
 
 
 
 
 
1 
 
 
 
 
 
daily 
 
 
 
 
 
2 
 
 
 
 
 
daily 
 
 
 
 
 
3 
 
 
x 
 
 
daily 
 
 
 
 
 
*Blood sample = baseline troponin, comprehensive metabolic panel (CMP) and complete blood count (CBC) at the screening visit, a sample 
prior to visit #2 (3-weeks after treatment #1), and 3 weeks after treatment #2. An additional blood sample will be obtained for genetic analysis 
for P-glycoprotein polymorphisms on the first study day; TQD=tariquidar; OND=ondansetron; NRS=numerical rating scale; NPSI=neuropathic 
pain symptom inventory; CPM=conditioned pain modulation; WUR=wind-up ratio; PK – blood samples for ondansetron pharmacokinetics; 
QST=quantitative sensory testing. 
** CSF sample can be obtained anywhere between 30 and 180 minutes after start of ondansetron infusion. 
 
2.4.6 QST protocol 
 
A QST battery will be applied for more comprehensive sensory characterization of these patients. The QST will 
follow a standard protocol,32 and include the assessment of mechanical detection threshold (MDT) with Semmes-
Weinstein monofilaments, mechanical pain thresholds (MPT) with standardized weighted metal probes, cold and 
warm detection thresholds (CDT and WDT), as well as cold and heat pain threshold (CPT and HPT) with TSA-II, and 
vibration detection threshold (with standard tuning fork). QST tests will be performed in the area of maximum pain 
and a control, non-painful area. 
 
Thermal detection and thermal pain thresholds 
Equipment: The Thermal Sensory Analyzer (TSA-II platform - Medoc, Ramat Yishai, Israel) will be used to determine 
thermal detection and pain thresholds. This equipment is used globally for functional assessment of pain and 
temperature-conducting nerve fibers (C and A-delta fibers). 
 
Method and Background: Using the thermal sensory analyzer, cold and warm detection thresholds (CDT and WDT, 
respectively), as well as cold and heat pain thresholds (CPT and HPT, respectively) will be determined.33,34 The 
thermode with contact area of 9.0 cm2 is applied to the tested site, and all thresholds are determined by 
continuous ramping of temperature from 32°C baseline temperature by 1°C/s until the subject presses the ‘stop‘ 
button. Cut-off temperatures are 0°C and 50°C, to minimize thermal damage to the skin. The baseline temperature 
to which the thermode returns before each test is 32°C. The average threshold is calculated from three 
measurements in each area. 
 
Determination of mechanical detection threshold (MDT) 
Equipment: A set of standardised Semmes-Weinstein monofilaments (0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 and 
256mN). The contact area of the filaments with the skin is of uniform size (<1 mm²) and texture. 
 
Methods and Background: Standardised Semmes-Weinstein monofilaments will be used in a modified “method of 
limits” manner using 3 series of increasing and decreasing stimulus intensities to determine the geometric average 
as the tactile detection threshold of the affected and unaffected skin areas 35. 
Monofilaments of different stimulus intensities are used to determine the tactile detection thresholds. A filament 
eliciting 16mN force* is applied first, followed by filaments of consecutively lower intensity until the patient cannot 
detect the stimulus being applied. This respective force represents the first threshold value. The order in which the 
stimuli are applied is then reversed and stimuli of consecutively greater intensity are applied until sensation is 
detected (this intensity becomes the second value). Again filaments with decreasing intensity are applied until in 
total 3 upper and lower values of detection are fulfilled from which the mechanical detection threshold can be 
determined. 

4/21/2022 
14 
 
* In case the first filament with an intensity of 16mN is not detected, the next highest intensity filament which can 
be detected must be used as a starting intensity. However, the relevant force of this stimulus is not documented. 
Filaments with consecutively lower intensity are applied until the patient cannot detect the stimulus being applied. 
The procedure is followed as above; until in total 3 upper and lower values of detection are fulfilled from which the 
mechanical detection threshold can be determined. 
 
Determination of mechanical pain thresholds (MPT) 
Equipment: A set of standardized weighted metal probes (Nervetest, MRC systems) exerting pressure of 8, 16, 32, 
64, 128, 256 and 512 mN. 
 
 
Methods and Background:  
The standardized metal probes will be used in a modified method of levels manner, 3 series of increasing stimulus 
intensities to detect the mechanical pain threshold. Beginning with an applied force of 8mN, stimuli increase in 
intensity until the sensation induced by increased pressure can be described as “sharp”. The corresponding force is 
used to represent the first MPT value. The stimuli are the applied in the descending order, until the sensation 
induced by pressure is described as “blunt”. The procedure is then repeated a total of 3 times and until a total of 6 
values are obtained, from which the mean MPT is determined.  
 
Determination of vibration detection thresholds (VDT) 
Equipment: A standard tuning fork. 
Methods and Background:  
The tuning fork will be applied at the lateral malleolus, and vibration detection threshold (the moment patient stops 
feeling the vibration of the tuning fork applied to the skin) will be measured on a scale of 1-8. The test will be 
repeated a total 3 times. VDT is determined as an average of the three measures.  
 
Determination of wind-up ratio (WUR) 
Equipment: A standardized weighted metal probe (Nervetest, MRC systems) exerting pressure of 256 mN. 
Methods and Background:  
Wind-up ratio (WUR) to punctate mechanical stimulation (i.e. temporal summation) will be measured as another 
surrogate of descending facilitation. WUR will be assessed by a published protocol.32,36 In this test a 256mN pinprick 
probe (Nervetest, MRC systems, Germany) is first applied singularly and then as a series of 10 identical stimuli with 
1Hz frequency within an area of 1 cm2. Immediately following the single and 10 stimuli, participants rate their pain 
on a 0-10 NRS, and a ratio is calculated using these values. This procedure is repeated twice; a geometric ‘wind-up’ 
average is calculated from the two ratios.  
 
Determination of conditioned pain modulation (CPM) 
Equipment: A thermostat-controlled digital water bath; The Thermal Sensory Analyzer (TSA-II platform - Medoc, 
Ramat Yishai, Israel) 
Methods and Background:  
Conditioned pain modulation (CPM) is a surrogate measure of descending inhibition vs. facilitation. CPM efficiency 
is derived from a patient report of pain intensity following application of a calibrated painful stimulus (called test 
stimulus) with and without another ongoing unpleasant stimulus (called conditioning stimulus). The intensity of the 
conditioning and pain stimulus will be determined individually by cold temperature that elicits pain intensity of 30-
70 on 0-100 NRS (conditioning), and hot temperature that elicits pain intensity of 60 on 0-100 NRS (test).  
The conditioning stimulus will be applied by immersing the non-dominant or non-affected arm in a cold thermostat-
controlled water bath. The initial temperature will be set to +8°C. If that temperature elicits pain intensity between 
30-70 on 0-100 NRS, the CPM protocol will use +8°C conditioning. Alternatively, water temperature will be adjusted 
in 2°C increments (lowest will be 2°C) to elicit pain in that range. 
The previously determined Pain-60 test stimulus will be applied at the dominant or affected forearm, using a 3x3 
cm Peltier thermode connected to TSA-sensory analyzer (Medoc Inc, Ramat Yishai, Israel).  

4/21/2022 
15 
 
The length of the conditioning is 60 seconds; during the last 30 seconds, the test stimulus is applied. The difference 
between test stimulus intensity with and without conditioning is defined as CPM magnitude. CPM>0 (i.e. increased 
pain to test stimulus following conditioning) implies descending pain facilitation.37,38  
 
 
2.4.7   Statistical Methods 
 
All demographic, sensory, analytical and patient-reported data will be captured on case report forms, manually 
entered to a Research Electronic Data Capture (REDCap) database that will be created for the project, and will be 
verified by an independent study team member. Plasma and CSF concentrations of ondansetron will be analyzed 
using a previously validated liquid chromatography method.39 Non-compartmental PK analysis will be performed to 
construct the concentration-time curves of ondansetron in human plasma with and without tariquidar.31 
Ondansetron concentration in CSF will be used to evaluate biodistribution of ondansetron to CNS, by the means of 
CSF to plasma concentration ratio. We do not expect missing outcomes data as all primary and most secondary 
outcomes will be collected with the participants physically present with the research team. Any missing data will be 
described, but no data imputation will be attempted. Intergroup comparisons for parametric data will be performed 
by paired t-test, and for non-parametric data by Wilcoxon signed-rank test for paired samples. McNemar’s test will 
be used to compare paired nominal data, and Fisher’s exact test to compare non-paired nominal data.  
 
Sample size: 
Change in spontaneous pain reduction (secondary outcome) was used to determine the sample size for the study. 
The baseline pain intensity (±SD) of patients with peripheral neuropathy we have recruited to various studies over 
the past two years have been 6.3±2.1. This proof of concept study expects to demonstrate a 30% difference in 
pain reduction between the two intervention sessions. Given that in a published study of IV ondansetron in NeuP 
a 20% pain reduction has been observed (although not different from placebo), we assume pain intensity of 
5.1+2.1 in ondansetron+placebo group, and expect pain intensity of 4.0+2.1 in ondansetron+tariquidar group at 
the 60-120 minute timeframe. To determine a significant difference with two-tailed α<0.05 and 85% power (effect 
size 0.52), 28 subjects will be required in this cross-over study. We expect to enroll 28-32 participants to the study 
to reach 28 evaluable participants. 
 
3.0 Management of Intercurrent Events 
 
3.1 Adverse Experiences 
The investigators will closely monitor subjects for evidence of systemic adverse events.  All adverse events will be 
reported and followed until satisfactory resolution.  The description of the adverse experience will include the time 
of onset, duration, severity, etiology, relationship to the study drug (none, unlikely, possible, probable, highly 
probable), and any treatment required. Participants will notify the monitoring physician/RN coordinator of any 
adverse experience during the study period. 
At discharge, subjects will be provided with contact information for team members and instructed to call if any 
adverse events occur. 
 
3.2 Dose Limiting Toxicities 
The dose-limiting toxicities (DLT) for the study are defined as follows: 
 
New QTc prolongation >60ms greater than baseline, or QTc>450ms   
 
Troponin T elevation > 0.03 ng/mL  
 
Syncope  
 
Sustained increase or decrease in SBP of >30 mmHg systolic or >20 mmHg diastolic 
 
Sustained vital sign abnormalities  
 
HR<40 or >120 beats per minute 
 
SpO2 <92% 

4/21/2022 
16 
 
 
ALT/AST ≥3xULN AND total bilirubin ≥2xULN 
 
AST/ALT ≥8xULN  
 
Altered mental status (delirium, inability to follow simple commands) 
 
Persistent visual disturbances 
 
If DLT is observed in >2 participants, ondansetron dose will be reduced to 12 mg for all new participants.  
If further DLT is observed in >2 participants receiving 12 mg ondansetron, the dose will be reduced to 8 mg for all 
new participants. 
 
3.3 Premature withdrawal from a study for a single patient 
If at any time the constraints of this protocol are considered to be detrimental to the patient’s health and/or the 
patient no longer wishes to continue protocol therapy, the participant will be withdrawn from the study and the 
reason(s) for withdrawal documented in the case report forms. 
 
Any participant will be withdrawn from the study in the following scenarios: 
 
Death 
 
Anaphylaxis 
 
Allergic reaction with urticarial, rash or dyspnea, 
 
Cardiac arrest 
 
Arial or ventricular fibrillation 
 
Ventricular Tachycardia 
 
Syncope 
 
AV block (2nd degree type II or 3rd degree) 
 
New QTc prolongation >60ms greater than baseline or >470ms for men or >480ms for women 
 
Troponin elevation > 0.03 ng/mL 
 
Sustained increase or decrease in SBP of >30 mmHg systolic or >20 mmHg diastolic 
 
Sustained vital sign abnormalities  
 
HR<40 or >120 beats per minute 
 
SpO2 <92% 
 
Temperature <36 C or >38°C 
 
 
ALT/AST ≥3xULN AND total bilirubin ≥2xULN 
 
AST/ALT ≥8xULN  
 
Thrombocytopenia 
 
Clinically meaningful CBC abnormalities (compared to baseline) 
 
Altered mental status (delirium, inability to follow simple commands) 
 
Seizures  
 
Persistent visual disturbances 
 
General or specific changes in the patient’s condition that render the patient unable to receive further 
treatment in the judgment of the investigator 
 
Suspected or confirmed pregnancy 
 
Serious non-compliance with the study protocol 
 
Lost to follow-up 
 
Patient withdraws consent 
 
Investigator removes the patient from study 
 
Investigator decides to close the study 
 

4/21/2022 
17 
 
If a participant withdraws from the study, the participant will be replaced in order to provide the required number 
of evaluable subjects. Subjects will be withdrawn if the investigator decides that discontinuation is in the best 
interest of the subject, or the subject requests withdrawal from the study. 
 
3.4 Adverse experiences and premature termination of the study 
 
If any patient should suffer any serious adverse effect, including but not limited to serious neurologic or 
cardiovascular dysfunction or a reaction concerning for anaphylactic or anaphylactoid reaction, patient recruitment 
and randomization will be halted until the etiology of such an adverse reaction can be determined. 
 
If two or more participants suffer any serious adverse effect, the study team will discuss premature termination of 
the study with Washington University IRB and the FDA. 
 
If more than 7 participants meet the early withdrawal criteria, due to serious or non-serious adverse effects, the 
study will meet the early stopping criteria.  
 
3.5 Potential Risks 
 
3.5.1 Potential risks for intravenous (IV) catheter placement 
Placement of the intravenous catheter can cause a brief, generally mild period of pain lasting for a few seconds at 
the site of catheter placement. There is the potential that the first attempt at venous cannulation will be 
unsuccessful and the patient will require multiple attempts to place the IV. Intravenous catheter placement can 
cause a bruise, though in patients without known bleeding diathesis this is usually mild and self-resolving. Some 
individuals experience dizziness or lightheadedness. Other complications are rarer. Infection from a cleanly-placed 
IV is rare, especially given the short anticipated duration of catheter retention. There is the chance of the catheter 
becoming dislodged from the vein and infiltrating the contents flowing through it into the muscle or subcutaneous 
fat. As only a minimal amount of fluid will flow through the vein and no caustic or vasoactive substances will be 
used (except in the case of a rare, potentially serious adverse event), this presents no risk to the volunteer. 
 
3.5.2 Potential risks from lumbar puncture 
Lumbar Puncture (LP) is widely used technique of CSF sample aspiration by thin needle from the subarachnoid space 
of the lumbar dural sac. LP is a major diagnostic tool used in neurological clinics as relatively safe method to obtain 
CSF sample for further analysis. The important complication of LP is post-lumbar puncture headache (PLPH) 
developing within a week after LP. In a multisite international trial, 9% of 3868 enrolled patients reported PLPH,40 
with 0.3% patients requiring and epidural blood patch (procedure of introducing autologous blood into epidural 
space). Other adverse events were non-specific headache (10.2%), back pain (17%), nausea and/or vomiting (2.5%), 
dizziness 45 (1.3%). Other possible complications may include bleeding as well as temporary or permanent 
neurological damage, although these are extremely rare. Management of these potential adverse events are 
described below in section 3.4.  
 
3.5.3 Potential risks from blood collection 
The total amount of blood collected during each study visit is approximately 60 mL ( about 10% of a standard blood 
donation), which should not have adverse physiological effects. As described above, the IV catheter placement 
necessary for blood draws may cause bleeding, bruising, pain, dizziness/lightheadedness, or (very rarely) infection.   
 
3.5.4 Potential risks from ondansetron 
Ondansetron is a widely-used drug approved for prevention of chemotherapy-induced, radiotherapy-
induced, and postoperative nausea and vomiting. It is commonly used off-label for treatment of nausea and 
vomiting. The most common reported side effects (~10%) include headache, constipation, and fatigue. Less 
common (<10%) include drowsiness, diarrhea, fever, anxiety, transient liver enzyme increases, or burning at 

4/21/2022 
18 
 
injection site. Rare side effects (<1%) include cardiac arrhythmias, ECG changes, dry mouth, anaphylaxis, transient 
blindness, and hypokalemia.  
The FDA has issued a drug safety communication regarding the risk of QTc prolongation and potential 
progression to torsades de pointes. The risk of cardiac side effects was determined to be dose-dependent with a 
maximum QTc prolongation by 20msec after 32mg IV administration; consequently, 32mg IV doses are no longer 
approved. 16mg IV infusion doses (as proposed in this study) are still approved and frequently used for 
chemotherapy-induced nausea and vomiting. Risk of arrhythmia is increased by comorbidities including: congenital 
long QT syndrome, congestive heart failure, bradyarrhythmias, or patients taking concomitant medications that 
prolong the QT interval. All such patients would be excluded by our proposed exclusion criteria. Additionally, in our 
recent study, none of the 15 subjects who received 16mg intravenous ondansetron (meeting the same 
inclusion/exclusion criteria as in this study), experienced arrhythmias or any other serious side effects 
(NCT02901054).  
 
3.5.5 Potential risks from tariquidar 
Tariquidar is an experimental drug, a specific 3rd generation inhibitor of P-glycoprotein, currently in 
development by Avaant Pharmaceuticals Inc. In a Phase I study in healthy volunteers, intravenous tariquidar 
doses in the range of 0.1-2mg/kg did not cause any adverse effects.41 
In healthy volunteers, a 2mg/kg intravenous dose resulted in no serious adverse effects. Two of ten 
subjects receiving this dose developed mild metallic taste during tariquidar infusion. Two subjects who received a 
4mg/kg dose experienced light-headedness and conjunctival infection. One subject developed a brief syncopal 
episode after a PET-scan procedure, resolved without sequelae. A 6mg/kg intravenous dose resulted in mild 
metallic taste and nausea that resolved after the infusion was stopped.42 
In healthy volunteers receiving 4, 6 or 8mg/kg intravenous tariquidar doses, no serious adverse events 
have been observed.43 Overall, four cases of hematoma and one case of phlebitis due to cannulation, as well as 
five cases of metallic taste in the mouth occurred. Other adverse drug effects to tariquidar were vertigo (two), 
bradycardia (one) and headache (one). Heart rate, blood pressure as well as ECG did not significantly change 
during and after tariquidar administration. No subject had any significant change in blood chemistries (including 
electrolytes, liver and renal function tests), complete blood count and coagulation over the whole study period. 
No volunteer discontinued prematurely the study. In one case, for safety reasons the infusion was stopped 20 min 
before the scheduled end due to moderate orthostatic hypotension and vertigo.43 
In an additional healthy volunteer study testing intravenous tariquidar doses in a range of 3-8 mg/kg, 
adverse events possibly/probably related to tariquidar included 1) dysgeusia (changes in the sense of taste) in 
6/20 subjects, across 3mg/kg, 4mg/kg, 6mg/kg and 8 mg/kg doses; 2) vertigo in 3/20 subjects receiving 3mg/kg 
(n=1) and 8mg/kg (n=2) doses; 3) headache (n=1, 6mg/kg dose); and 4) hypotension and bradycardia (n=1, 
8mg/kg; infusion stopped after 7.2mg/kg was infused). 
Seven clinical studies of the pharmacokinetics and efficacy of tariquidar have been completed in cancer 
patients. The main use of tariquidar in these clinical trials has been to enhance BBB-penetration and tumor-
penetration of Pgp-substrate chemotherapy agents, to improve the efficacy of cancer chemotherapy. In these 
studies, tariquidar have been shown to increase the severity of known side effects of chemotherapeutic agents, 
including hematological abnormalities such as anemia and thrombocytopenia. No additional side effects other 
than those known to be associated with chemotherapy have been identified in patients who have received 
tariquidar (Tariquidar Investigator’s Brochure).  
Each ampoule of Tariquidar for injection contains 10 mL of 7.5 mg/mL tariquidar (as the dimesylate salt, in 
a sterile solution of 20% ethanol/80% propylene glycol with a small amount of hydrochloric acid) for a total of 75 
mg tariquidar free base per ampoule. Ampoules will be stored refrigerated (2-8°C) and protected from light. Within 
4 hours prior to administration, the tariquidar solution will be diluted aseptically by the Washington University 
Investigational Drug Service with 5% dextrose in water (D5W) to give a final volume of 250 mL. The diluted solution 
will be protected from light until infusion is complete. 
 
3.5.6 Potential risks from tariquidar and ondansetron combination 
 

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19 
 
Combined use of tariquidar and ondansetron may change the safety profile of each of drugs. Changes in 
ondansetron pharmacokinetics such as increased penetration to CNS, is an expected effect of tariquidar. However, 
there is a potential risk of increased incidence of undesirable effects of ondansetron such as drowsiness, diarrhea, 
fever, anxiety, transient liver enzyme increases, prolonged QTc, arrhythmias.   
Preliminary data from our ondansetron-tariquidar combination study in healthy volunteers suggest no 
occurrence of arrhythmia or clinically significant QTc prolongation. Most of adverse events were not related with 
study drugs, but to intrathecal catheterization.  QTc changes ater Ondansetron and Ondansetron+ Tariquidar 
administrations were insignificant: mean QTc change was -2.5 ms (±16.1) after Day 1 and 2.9 ms (±12.9) after Day 
2, p=0.67 (see Table 2).  
 
 
Table 2. QTc values (ms) of subjects on Day 1-3 
Subject 
Ondansetron dose (mg) 
Day 1 
Day 2 
Day 3 
1 
4 
390 
404 
417 
2 
4 
434 
414 
415 
3 
8 
424 
430 
425 
4 
8 
412 
420 
405 
5 
8 
399 
400 
399 
6 
8 
430 
397 
424 
7 
16 
401 
397 
406 
8 
16 
395 
403 
397 
 
3.5.7 Potential risks from thermal testing 
Risk of injury related to thermal pain testing is minimal.  Thermal testing is widely used and considered safe.  While 
thermal testing does produce pain, risks to the individual are minimal, because 1) the pain is transient in nature and 
generally subsides immediately after the procedure; 2) subjects are instructed that they may stop any procedure at 
any time with no adverse consequences; and 3) the level of pain experienced by subjects is below their tolerance level.  
With thermal stimulation there is a very slight risk of a burn, but this is minimized by the following: 1) positive lockout 
of stimulus parameters above 52°C; and 2) the stimulator has built in a shut-down system to prevent the delivery of 
prolonged or high intensity stimuli.  The TSA-II thermal devices have FDA 501(k) clearance (K922052). CPM testing is 
conducted using an ice water bath at 8°C and exposure is limited, thereby posing minimal risk of tissue damage from 
cold exposure. 
 
3.6 Procedures to Minimize Potential Risks 
Drug infusions will be administered in the Washington University Clinical Research Unit under the supervision of 
the PI and the co-investigators. Subjects will be monitored by study personnel during infusions, and dedicated RN 
personnel is available 24-hours a day on site.  
Inclusion and exclusion criteria, monitoring, and the clinical protocol are designed to ensure that risks are absolutely 
minimal. Participants are informed that participation is voluntary and they may refuse to participate and may 
withdraw from the study at any time without penalty. A pregnancy test will be performed on women of childbearing 
potential and subjects excluded if pregnant. Subjects will be told that in the event of a physical injury as the direct 
result of study procedures, they will be cared for by a member of the investigating team at no cost, within the limits 
of the Washington University compensation plan. 
Subjects will be instructed at the beginning of the visit that they can alert the investigator any time they experience 
bothersome side effects. In particular, they will be instructed to immediately report palpitations or 
dizziness/lightheadedness. Continuous physiologic monitoring (ECG, pulse oximetry, and intermittent non-invasive 
blood pressure) will be used from the beginning of drug infusion until 2 hours after ondansetron infusion is 
complete. Medical personnel will be immediately available throughout the visit. The subject can voluntarily 
withdraw from the study at any time. 
With regard to confidentiality; 1) all subjects will be assigned a study ID number, 2) Samples will be kept 

4/21/2022 
20 
 
confidentially.  They will be coded, with a key to the code linking code numbers to names kept at a separate location, 
under lock and key.  3) The link to identifiers will be destroyed at the end of the study. 4) Data will be stored under 
lock and key (office, file cabinet) and only the investigators and research team will have access.  If data are 
published, there will be no link to identifiers.  Study data will not be revealed to any organization, individuals other 
than the subjects, or the subjects themselves. 5) Study data will not be entered in subjects' medical records.  
 
3.6.1 Prevention, diagnosis and management of ondansetron-associated arrhythmia 
To minimize the risk of ondansetron-associated arrhythmia, subjects will have a 12-lead ECG performed at 
the screening visit. Subjects with QTc>450msec or any arrhythmia will be excluded. Potential subjects that would 
be at elevated risk of developing ondansetron-associated arrhythmia (e.g., current use of QT-prolonging 
medication) are ineligible for this study. The ondansetron dose used (16mg IV infused over 15min) is approved and 
commonly used with chemotherapy. Subjects will be monitored with telemetry from the beginning of drug infusion 
until 2 hrs after infusion is complete. If arrhythmias or prolonged QT develop at any time during the study, subjects 
will be immediately assessed by a physician. If ECG abnormalities are mild (e.g., infrequent premature 
atrial/ventricular contractions or QTc>450ms but <500ms), subjects will be monitored in the pain management 
center by nursing and medical personnel for at least 4 hrs. If the signs/symptoms meet the severity criteria 
described in Section 3.2 (“Premature discontinuation of protocol for a single patient”), the patient will be withdrawn 
from the study and sent to the Barnes-Jewish Hospital emergency department (ED) for further evaluation, including 
cardiology consultation or hospital admission, as needed. 
 
3.6.2 Prevention, diagnosis and management of post-lumbar puncture headache (PLPH) 
Lumbar puncture and sampling procedure will conducted in a way to minimize the effect of risk factors 
contributing PLPH development. Particularly, an atraumatic needle of 25 G diameter will be used for LP procedure. 
Passive withdrawal of CSF is known as associated with a lower risk for headache, therefore active withdraw with 
syringe will be avoided left only for rare cases. 
 Subjects will be asked to notify clinical personnel immediately if they develop a headache while in the 
clinical research unit. Additionally, the periodic nursing assessment (see supplementary document) will include 
questions about headache. If a participant endorses new-onset headache, the anesthesiologist will be notified. The 
anesthesiologist will perform a brief history and physical exam to determine whether or not the symptoms suggest 
PLPH. If PLPH is likely and symptoms are mild, conservative treatment will be initiated. The participant will be 
encouraged to lie in a comfortable position and drink fluids. The subject will also be offered analgesics including 
acetaminophen/butalbital/caffeine (e.g. Fioricet®), ibuprofen, or acetaminophen alone. IV fluids or medication will 
be reserved for subjects who cannot tolerate oral administration due to nausea/vomiting. If symptoms are severe 
(e.g. uncontrollable nausea/vomiting, severe subject distress), the subject will be removed from the study and 
epidural blood patch will be offered (procedure described below).  
Discharge paperwork will include clear descriptions of PLPH and instructions about whom to contact in the 
case of headache. If a subject reports likely PLPH by contacting study personnel or during a follow-up phone call, 
symptom severity will be assessed. Conservative treatment (as described above) will be offered for the first 24-48 
hrs. If unresolved, the participants will be brought to the Clinical Research Unit for further evaluation or treatment 
per discretion of the investigators which could include a potential epidural blood patch procedures, performed by 
the anesthesiologists on the study team. 
Epidural blood patches are commonly performed by our Acute Pain Service. Sterile technique (including 
chlorhexidine swabs, caps, masks, and sterile gloves) is used at all steps by all personnel involved. First, the epidural 
space is located using a standard Tuohy needle and loss-of-resistance technique. Next, 20ml of the subject’s blood 
is drawn and then injected through the Tuohy needle.  Injection is stopped if the subject complains of pressure or 
paresthesia that could indicate compression of the cauda equina.  
 
3.6.3 Prevention of drug-drug interactions between study drugs and concomitantly used antidepressants 
 
Citalopram, Escitalopram and Duloxetine are not known to produce clinically meaningful interactions when 
combined with potent P-gp or CYP3A4 inhibitors. When tested specifically in combination with P-gp inhibitors such 

4/21/2022 
21 
 
as simeprevir, cyclosporine, and verapamil, meaningful interactions were NOT identified for escitalopram and 
duloxetine44-46.  Given that tariquidar is a selective P-gp inhibitor (and possibly weak CYP3A4 inhibitor), we do not 
expect any meaningful interactions with citalopram, escitalopram, or duloxetine. 
 
Table 3 outlines the potential of drug interactions between SSRI/SNRI antidepressants in the context of use with P-
gp inhibitors, and served as a basis for inclusion/exclusion of particular SSRI/SNRI antidepressants. 
 
Table 3. Potential of drug interactions between SSRI/SNRI antidepressants and P-gp inhibitors. 
SSRI 
 
FDA-
approved 
maximum 
dose 
(mg/day) 
Known P-
gp 
substrate 
Known CYP3A4 
substrate 
Known / 
Clinically 
relevant 
interactions 
with other P-
gp inhibitors 
Clinically-
relevant 
interactions with 
CYP3A4 
inhibitors 
Comments on 
QT interval 
prolongation 
Citalopram 
40 
No47,48 
Yes, CYP3A4 
and CYP2C1949 
None found 
(extensive 
search in 
PubMed, Lexi-
Comp, and 
IBM 
Micromedex) 
No. ketoconazole 
co-
administration 
did not 
significantly 
affect the 
pharmacokinetics 
of citalopram [1] 
Dose-
dependent QT 
prolongation 47 
50 
Escitalopram 
20 
No48 51 
Yes, CYP3A4, 
CYP2D6, and 
CYP2C19 
None found 
(extensive 
search in 
PubMed, Lexi-
Comp, and 
IBM 
Micromedex)49 
No. 
Administration of 
ritonavir (potent 
CYP3A4 inhibitor) 
did not affect the 
pharmacokinetics 
of Escitalopram 
52 
No clinically 
relevant QT 
prolongation vs 
placebo 52 
Paroxetine 
60 
Possibly53 
54. 
However, 
may be a 
weak 
inhibitor of 
P-gp 55 
No. CYP2D6 
and unchanged 
excretion to 
urine 
No 
No. Also, 
paroxetine does 
not meaningfully 
inhibit CYP3A456  
Likely rare, only 
a few cases 
reported 57 
Fluoxetine 
80  
Conflicting 
data 53,54 58 
Yes. Also 
possible 
inhibitor of CYP 
3A4. Although 
overall CYP3A4 
inhibition with 
fluoxetine is 
likely not 
clinically 
meaningful 44, 
some potential 
interactions 
have been 
reported with 
CYP3A4 
substrates.59 
No 
Verapamil (likely 
due to 
Fluoxetine’s 
inhibition of 
verapamil 
metabolism 44) 
Fluoxetine has 
been associated 
with potential 
increase in risk 
of QT interval 
prolongation 50  

4/21/2022 
22 
 
Sertraline 
200 
No60,61. 
However, 
may be a 
weak 
inhibitor of 
P-gp55 
Unclear. 
Multiple 
sources of data 
suggest 1st pass 
metabolism 
and N-
demethylation, 
but specific 
enzymes are 
not well 
understood60,61, 
possibly 
suggesting 
CYP2C19 and 
CYP2B6 
None found 
(extensive 
search in 
PubMed, Lexi-
Comp, and 
IBM 
Micromedex) 
No 
Likely not 
significant62 
SNRI 
FDA-
approved 
maximum 
dose 
(mg/day) 
Known P-
gp 
substrate 
Known CYP3A4 
substrate 
Known / 
Clinically 
relevant 
interactions 
with other P-
gp inhibitors 
Clinically-
relevant 
interactions with 
CYP3A4 
inhibitors 
Comments on 
QT interval 
prolongation 
Duloxetine  
120 
No63,58 
No. 
Metabolized 
primarily by 
CYP1A2 and 
CYP2D6 
None found 
(extensive 
search in 
PubMed, Lexi-
Comp, and 
IBM 
Micromedex)49  
None found 
No clinically 
relevant QT 
prolongation vs 
placebo 63 
Venlafaxine 
225 
Possibly 
53,54 
Yes. Also 
metabolized by 
CYP2D6 
(major). 
Venlafaxine 
does not 
meaningfully 
inhibit CYP3A4 
64 
Interaction 
with 
ketoconazole, 
likely due to 
CYP3A4 rather 
than P-gp 
inhibition. 
Minimum 
interference 
with P-gp 
activity in vitro 
64 
Clinically 
meaningful 
interaction with 
ketoconazole, 
likely because 
3A4 inhibition 46 
QT prolongation 
rare, likely 
happens only 
with very high 
doses / 
overdose65,66 
Desvenlafaxine 
400mg, but 
doses >50mg 
not 
recommended 
No67  
Yes (minor). 
Also excreted 
renally 
unchanged 
(45%). 
Desvenlafaxine 
does not 
meaningfully 
inhibit CYP3A4 
64 
Unlikely to be 
affected67 
Ketoconazole can 
increase plasma 
concentrations of 
desvenlafaxine. 
Potential 
interaction with 
potent CYP3A4 
inhibitors. 
No clinically 
relevant QT 
prolongation vs 
placebo67 
 
 
 

4/21/2022 
23 
 
For patient safety, we will limit maximally allowable doses of citalopram, escitalopram and duloxetine in enrolled 
participants to 50% of FDA-approved maximum dose, eg, 20mg for citalopram, 10mg for escitalopram and 60mg 
for duloxetine. 
Due to possible interactions with other SSRI/SNRI medication, we will exclude patients taking other SSRI/SNRI 
medications such as paroxetine, fluoxetine, sertraline, venlafaxine, and desvenlafaxine. 
 
3.6.4 Prevention, diagnosis and management of other lumbar puncture-related complications  
The potential complication of epidural hematoma is exceedingly rare in patients with normal coagulation40. 
To minimize the risk of epidural hematoma, subjects will be asked over the phone and at the in-person screening 
visit about a history of clotting disorders, easy bleeding/bruising, liver disease, or use of anticoagulants. A complete 
blood count will be drawn during the screening visit to rule out thrombocytopenia. Any potential subject with 
abnormalities on history or lab work will be excluded from the study.  
To minimize the risk of infection or nerve damage from lumbar puncture, the procedure will be done by 
experienced anesthesia personnel (board-certified or board-eligible anesthesiologists) under sterile conditions. 
Specifically, the subject’s lumbar region will be sterilized with a chlorhexidine swab, a sterile drape will be placed, 
the anesthesiologist will wear sterile gloves, all personnel in the room will wear a cap and mask, and a dressing will 
be applied after LP. Subjects will be instructed to notify the anesthesiologist of the occurrence and resolution of 
any paresthesias during LP procedure. If symptoms are severe (e.g., severe pain, motor deficits) the subject will be 
sent to the BJH ED for further evaluation including neurology consultation, imaging, or hospital admission, as 
needed. Subjects with temporary, minor paresthesias during placement may continue with the study.  
 
 
3.7   Data and Safety Monitoring Plan 
In compliance with the Washington University Institutional Data and Safety Monitoring Plan, and 
commensurate with the risks of the proposed study, we will use an Independent Data Safety Monitoring Board 
(DSMB) for data safety and monitoring for this study. The potential risks are attributable to the 5-HT3R antagonist 
ondansetron, and the Pgp efflux transporter inhibitor tariquidar. The Independent DSMB will consist of an 
anesthesiologist, a Neurologist, and an independent Biostatistician. This DSMB will review the annual summary of 
adverse events, prior to data and safety monitoring report submission to the Washington University IRB. In addition, 
the board will review all reports of a Serious Adverse Event, or an Unexpected Adverse Event.  
The study principal investigator and Research Patient Coordinator will monitor for serious toxicities on an 
ongoing basis. Once the principal investigator or Research Patient Coordinator becomes aware of an adverse event, 
the AE will be reported to the HRPO according to institutional guidelines. 
 
 
4.  HUMAN SUBJECTS RESEARCH 
 
4.1 Protection of Human Subjects 
The study will be conducted under appropriate Washington University Institutional Review Board protocols and 
consent forms approvals.  The study will be conducted under the supervision of the PI, who is a GCP-certified 
pharmacist with several years of experience in the conduct of human studies; three board-certified 
anesthesiologists pain management specialists as co-investigators, all of them with extensive clinical experience 
and research experience.  
 
4.2 Sources of Materials 
Subjects will be recruited from the greater Saint Louis area via their Washington University Pain Management 
Center physicians, flyers, IRB-approved Epic direct recruitment tools, and the Volunteer for Health organization. 
Data on comorbidities and concomitant medication use are provided by subjects. Specimens include blood obtained 
exclusively for determining ondansetron concentration, and an additional blood sample is obtained for determining 

4/21/2022 
24 
 
single nucleotide polymorphisms in P-gp and OCT1 transporters. Other data including quantitative sensory testing 
are obtained exclusively for research purposes. 
 
4.3 Recruitment and Informed Consent 
Participants will be recruited through their Washington University Pain Management Center physicians, flyers, and 
through the Volunteers for Health organization. Interested subjects will contact the investigators. We may also 
screen the clinic schedule of the Washington University Pain Management Center, to identify potentially eligible 
subjects. We will then approach or contact them to ask for their interest in the study. Subjects will be given verbal 
(initially) and then written descriptions of the study aims, procedures, risks, and benefits, and will be required to 
give written informed consent. A member of the study team provides all study descriptions, informed consent, and 
answers all questions. Subjects are informed verbally and in writing that participation is voluntary and they may 
refuse to participate and may withdraw from the study at any time without penalty. 
 
4.4 Potential Benefits of the Proposed Research to the Subjects and Others 
Study subjects could potentially experience improvement in neuropathic pain. Society could benefit from 
development of a novel treatment for neuropathic pain. 
 
4.5 Inclusion of Women 
Both women and men suffer from neuropathic pain and thus will be included in this study.  
 
4.6 Inclusion of Minorities 
All of our studies actively encourage the participation of minorities in the research.  Our minority recruiting typically 
matches the demographic composition of the Washington University community from which subjects will be 
recruited (78% white, 21% Black, <1 % Hispanic).  
 
4.7 Inclusion of Children 
Children <18 years of age will not be studied in this investigation. Neuropathic pain is uncommon in this age group, 
and including children may expose them to an unnecessary risk without the benefit of generalizability of the results. 
 
 
 

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25 
 
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