Does a Reduced Response to Botulinum Toxin Mean “Resistance”?

Not necessarily.

A patient who experiences less benefit from botulinum toxin type A after previously responding may have secondary non-response, but this clinical observation does not automatically establish immunologic resistance.

Possible explanations include:

• incorrect or suboptimal muscle selection;

• injection placement or technique;

• insufficient dose or dose distribution;

• changes in anatomy or disease pattern;

• timing of outcome assessment;

• unrealistic treatment expectations; and

• development of neutralizing antibodies.

Neutralizing antibodies are therefore one possible explanation for reduced clinical effect, not the only explanation. [1–3]

What Is Secondary Non-Response?

Secondary non-response generally describes a situation in which a patient initially experiences clinical benefit from BoNT treatment but subsequently develops a partial or complete loss of treatment effect. [1]

This differs from primary non-response, in which adequate clinical benefit is not observed from the beginning of treatment.

Secondary non-response is a clinical description.

It does not identify its underlying cause.

What Are Neutralizing Antibodies?

Because botulinum neurotoxin is a protein, the immune system can recognize it and generate antibodies.

Not all antibodies have the same clinical significance.

Neutralizing antibodies, or NAbs, are antibodies capable of interfering with the biological activity of the neurotoxin.

If clinically relevant NAbs are present, they may reduce or prevent the expected pharmacologic effect of treatment. [1]

This phenomenon is often referred to as immunoresistance.

However, antibody detection and clinical response are not perfectly correlated.

Are All Botulinum-Toxin Antibodies Neutralizing?

No.

Antibody assays may detect antibodies that bind to components of a product without necessarily neutralizing the biological activity of the neurotoxin.

This distinction is important.

The clinically relevant question is not simply whether an antibody can be detected, but whether the immune response interferes with the pharmacologic action of the toxin.

Different antibody assays also measure different aspects of immune response and are not necessarily directly comparable. [1]

How Common Are Neutralizing Antibodies?

There is no single universal percentage.

Reported NAb prevalence varies substantially according to:

• indication;

• BoNT product;

• dose;

• duration of treatment;

• number of treatment cycles;

• study population;

• assay method; and

• whether investigators studied all treated patients or specifically selected secondary non-responders. [1,2]

A systematic review and meta-analysis incorporating 61 studies and 8,525 patients found major differences in NAb prevalence between clinical populations. [2]

Importantly, approximately half of patients classified as secondary non-responders in that analysis did not have detectable neutralizing antibodies.

This illustrates why loss of clinical response should not automatically be labeled antibody resistance. [2]

Can a Patient Have Neutralizing Antibodies and Still Respond?

Yes.

The relationship between NAbs and clinical effect is complex.

Some patients with detectable antibodies may retain clinical response, while some patients with secondary non-response have no detectable NAbs. [1–3]

For example, an analysis of 503 selected secondary non-responders found NAbs in 224 patients, or 44.5%. [3]

Therefore, more than half of the selected secondary non-responders in that study were antibody-negative.

The result reinforces the need to investigate non-immunologic causes of reduced treatment effect.

What Factors Have Been Associated With Immunogenicity?

Published literature has discussed several potential treatment-related factors, including:

• higher doses per treatment cycle;

• higher cumulative exposure;

• repeated treatment;

• short intervals between treatment cycles;

• booster injections; and

• product-related antigenic characteristics. [1]

However, these associations are not equally established in every indication or with every product.

Some studies have not confirmed simple dose- or interval-based relationships.

Therefore, these factors should be described as potential contributors rather than universal predictors of antibody development.

Why Might Therapeutic and Aesthetic Use Have Different Immunogenicity Considerations?

Therapeutic indications may require substantially larger doses and repeated treatment over many years.

Aesthetic treatment may use lower doses in many settings, although treatment patterns vary.

Because exposure differs across indications, immunogenicity data from one population should not automatically be transferred to another.

This is one reason reported NAb prevalence varies considerably across clinical indications. [1,2]

How Are Neutralizing Antibodies Tested?

Several methods have been used.

Laboratory approaches can broadly include:

binding assays, which detect antibodies capable of binding to toxin-related antigens;

and

functional or bioassays, which evaluate whether antibodies interfere with biological toxin activity. [1]

Historically, mouse-based bioassays have been used to evaluate neutralizing activity.

Different assays differ in sensitivity, specificity and biological relevance.

Therefore, an antibody result should always be interpreted together with the test method and clinical picture.

What Did the Neuronox/Meditoxin Blepharospasm Trial Test?

The randomized comparative trial of Meditoxin versus Botox in essential blepharospasm specifically included botulinum toxin type A antibody testing. [4]

The publication identifies Meditoxin as another name for Neuronox.

Antibody testing was performed using a mouse diaphragm assay.

Serum samples were collected before treatment and again after the clinical effect of BoNT-A had disappeared.

Results were reported qualitatively as positive or negative. [4]

This is notable because the study did not assess efficacy and routine adverse events alone; it also incorporated an immunogenicity-related laboratory assessment.

What Did the Neuronox Trial Find?

All 60 participants were tested initially.

At the final visit, 58 participants underwent repeat antibody testing. [4]

The study reported:

No positive botulinum toxin antibody tests.

This is a favorable study-specific finding for Meditoxin/Neuronox under the conditions of the trial.

However, the investigators explicitly cautioned against overinterpreting it.

They noted that the doses used might not have been high enough and that the follow-up period might not have been long enough to observe antibody formation. [4]

Can We Say “Neuronox Does Not Cause Antibodies”?

No.

The study supports the statement:

No positive antibody tests were detected in the Meditoxin/Neuronox blepharospasm trial under the study conditions and follow-up.

It does not support:

“Neuronox never causes antibodies.”

A trial involving approximately 60 patients and limited follow-up cannot exclude a rare or long-term immunogenic response.

This distinction is particularly important when developing product claims.

Can We Say Neuronox Has “Low Immunogenicity”?

That claim requires caution.

A single randomized trial with no positive antibody tests provides useful product-specific evidence, but it is not sufficient by itself to quantify the long-term immunogenicity rate of Neuronox across indications.

A robust claim of low immunogenicity would ideally require larger product-specific datasets involving:

• repeated treatment cycles;

• longer follow-up;

• defined antibody assays;

• clinically relevant populations; and

• assessment of the relationship between antibody status and treatment response.

The current evidence should therefore be described precisely rather than converted into a broader marketing claim.

What Did the Cerebral-Palsy Neuronox Trial Report?

The randomized multicentre study comparing Neuronox with BOTOX in children with cerebral palsy also included antibody assessment. [5]

The publication reported that no participant had a positive antibody assay before or after the intervention.

This adds another product-specific negative antibody finding.

However, as with the blepharospasm trial, it remains a study-specific observation and does not establish lifetime or universal absence of immunogenicity.

Why Is “Botox Resistance” Often an Oversimplification?

The phrase is commonly used to describe a treatment that appears to have stopped working.

Scientifically, however, several different situations may be hidden behind that phrase.

A reduced effect might result from:

wrong target

insufficient dose

changed anatomy

different treatment expectations

progression of the underlying condition

technical issues

or

neutralizing antibodies

Calling all reduced response “resistance” can therefore lead to an incorrect explanation of the problem.

What Should Be Evaluated When Treatment Seems Less Effective?

A clinical reassessment may consider:

1. whether the original diagnosis and treatment goal remain appropriate;

2. which muscles were selected;

3. dose and dose distribution;

4. injection technique and localization;

5. treatment interval;

6. previous product and treatment history;

7. whether the expected endpoint was assessed at an appropriate time;

8. changes in disease severity or anatomy; and

9. whether immunologic resistance is clinically plausible.

Antibody testing is therefore one component of an evaluation rather than an automatic first explanation for every weak response.

Does Switching Products Automatically Solve Secondary Non-Response?

Not necessarily.

If the reduced response is caused by incorrect targeting, insufficient dosing or another non-immunologic factor, changing products may not address the underlying problem.

If clinically relevant neutralizing antibodies are suspected or confirmed, management becomes more complex and should be guided by an appropriately qualified clinician.

The evidence should not be reduced to a universal recommendation to switch brands.

What Does the Evidence Say About Repeated Treatment?

Repeated exposure is relevant to immunogenicity because BoNT is a protein-based biologic therapy.

Literature has associated immunogenicity risk with treatment characteristics such as repeated cycles, higher doses and shorter intervals in some settings. [1]

However, evidence is heterogeneous.

For example, a study of 17 patients exposed to relatively high cumulative BoNT-A doses over long treatment periods did not detect neutralizing antibodies, demonstrating that proposed risk factors do not predict antibody formation with certainty. [6]

Risk should therefore be discussed probabilistically rather than as an inevitable consequence of repeated treatment.

What Does Historical Evidence Show About Antibody Status?

Sankhla, Jankovic and Duane described patients with dystonia who had become antibody-positive and clinically unresponsive but later reverted to antibody-negative status. [7]

Some patients again responded after BoNT-A treatment when antibodies were no longer detectable, while several subsequently redeveloped antibodies and lost benefit again.

This historical evidence illustrates that antibody status and clinical response can change over time.

It also reinforces the biological plausibility of antibody-mediated resistance while showing that the phenomenon is more complex than a permanent binary state.

Why Is Product-Specific Evidence Important?

Different BoNT products may differ in formulation, manufacturing and other characteristics relevant to immunogenicity. [1]

For this reason, class-level information cannot automatically establish the immunogenicity rate of an individual product.

For Neuronox, the evidence set includes randomized clinical studies in which antibody testing was incorporated, including the essential blepharospasm and cerebral-palsy studies. [4,5]

These results are useful because they are product-specific observations.

Their limitations must remain attached to them.

What Can Be Said About Neuronox Based on Current Evidence?

Evidence-based wording includes:

“Published Neuronox/Meditoxin clinical trials have included antibody testing.”

“No positive antibody tests were reported in the randomized essential blepharospasm trial.”

“No positive antibody assays were reported before or after intervention in the randomized cerebral-palsy trial.”

These statements accurately reflect the published studies. [4,5]

Claims such as:

“Neuronox does not cause resistance”

or

“Neuronox cannot produce neutralizing antibodies”

go beyond the available evidence.

Bottom Line

Reduced response to Botulinum Toxin Type A should not automatically be called antibody resistance.

Secondary non-response has multiple possible explanations, and neutralizing antibodies are only one of them.

Systematic evidence shows that a substantial proportion of patients with secondary non-response do not have detectable NAbs. [2,3]

For Neuronox, product-specific randomized trials have included antibody testing.

In the essential blepharospasm trial, 60 patients were tested initially and 58 were tested at the final visit, with no positive antibody tests reported. [4]

The randomized cerebral-palsy trial likewise reported no positive antibody assays before or after treatment. [5]

These are useful product-specific findings, but they do not establish that Neuronox is incapable of inducing antibodies or resistance.

The scientifically appropriate conclusion is:

Neuronox has published clinical studies containing product-specific antibody assessments in which no positive antibody findings were reported under the conditions and follow-up of those trials.

References

1. Bellows S, Jankovic J. Immunogenicity associated with botulinum toxin treatment. Toxins (Basel). 2019;11(9):491. doi:10.3390/toxins11090491. PMID:31454941.

2. Fabbri M, Leodori G, Fernandes RM, et al. Neutralizing antibody and botulinum toxin therapy: a systematic review and meta-analysis. Neurotox Res. 2016;29(1):105–117. doi:10.1007/s12640-015-9561-5. PMID:26467676.

3. Lange O, Bigalke H, Dengler R, Wegner F, deGroot M, Wohlfarth K. Neutralizing antibodies and secondary therapy failure after treatment with botulinum toxin type A: much ado about nothing? Clin Neuropharmacol. 2009;32(4):213–218. doi:10.1097/WNF.0b013e3181914d0a. PMID:19620852.

4. Yoon JS, Kim JC, Lee SY. Double-blind, randomized, comparative study of Meditoxin versus Botox in the treatment of essential blepharospasm. Korean J Ophthalmol. 2009;23(3):137–141. doi:10.3341/kjo.2009.23.3.137. PMID:19794937.

5. Kim K, Shin HI, Kwon BS, Kim SJ, Jung IY, Bang MS. Neuronox versus BOTOX for spastic equinus gait in children with cerebral palsy: a randomized, double-blinded, controlled multicentre clinical trial. Dev Med Child Neurol. 2011;53(3):239–244. doi:10.1111/j.1469-8749.2010.03830.x. PMID:21087238.

6. Bakheit AMO, Liptrot A, Newton R, Pickett AM. The effect of total cumulative dose, number of treatment cycles, interval between injections, and length of treatment on the frequency of occurrence of antibodies to botulinum toxin type A in the treatment of muscle spasticity. Int J Rehabil Res. 2012;35(1):36–39. doi:10.1097/MRR.0b013e32834df64f. PMID:22108625.

7. Sankhla C, Jankovic J, Duane D. Variability of the immunologic and clinical response in dystonic patients immunoresistant to botulinum toxin injections. Mov Disord. 1998;13(1):150–154. doi:10.1002/mds.870130128. PMID:9452341.

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