mug183 Antibody

What is MUC18 and why is it a significant target for melanoma therapy?

MUC18, also known as MCAM or CD146, is a cell surface glycoprotein that has emerged as a significant target in melanoma research. MUC18 expression correlates with tumor thickness and metastatic potential of human melanoma cells in nude mice . Moreover, ectopic expression of MUC18 in primary cutaneous melanoma cells leads to increased tumor growth and metastasis in vivo . This makes it a compelling target for therapeutic intervention.

The importance of MUC18 as an antibody target stems from several key characteristics. First, it is overexpressed in multiple melanoma subtypes, including cutaneous, mucosal, acral, and uveal melanoma . Second, it has predominated cell surface localization, making it accessible to antibody-based therapeutics . Third, MUC18 is overexpressed in both tumor cells and tumor-infiltrating blood vessels across major melanoma subtypes, making it a potential dual-compartment and universal melanoma therapeutic target .

How do MUC18 antibodies work differently from other melanoma-targeting approaches?

MUC18 antibodies represent a distinct therapeutic approach compared to other melanoma-targeting strategies, particularly in their mechanism of action and potential for dual targeting. Unlike conventional melanoma-targeting antibodies against targets such as glycoprotein NMB (GPNMB) and endothelin B receptor (ETRB), MUC18 antibodies can simultaneously target both tumor cells and tumor vasculature .

Previous melanoma-targeting ADCs against GPNMB and ETRB did not advance to gain regulatory approval due to limited efficacy and dose-limiting toxicities . Both ADCs used potent microtubule inhibitor payload monomethyl auristatin E (MMAE) . One key limitation was that most GPNMB and ETRB protein expressions were not predominantly on the cell surface, which may have negatively impacted ADC function .

In contrast, MUC18 antibodies such as ABX-MA1 have been shown to disrupt multiple tumor-promoting processes:

• They inhibit homotypic interactions between melanoma cells, as demonstrated in spheroid formation assays

• They disrupt heterotypic interactions between melanoma cells and vascular endothelial cells (HUVECs, which are MUC18 positive)

• They inhibit matrix metalloproteinase 2 (MMP2) promoter and collagenase activity, reducing tumor invasion capacity

• They directly interfere with angiogenesis by disrupting tube-like formation by endothelial cells

When developed as antibody-drug conjugates like AMT-253, MUC18 antibodies also deliver potent cytotoxic payloads specifically to tumor cells, inducing DNA damage and apoptosis while generating bystander killing effects .

What methods are used to validate MUC18 antibody specificity?

Validating the specificity of MUC18 antibodies is crucial for ensuring reliable research and therapeutic development. The following complementary approaches are commonly employed to confirm antibody specificity:

Immunoprecipitation (IP) followed by Mass Spectrometry (MS) is a gold standard approach. In this method, membrane extractions from MUC18-positive cell lines are used for IP with the test antibody, with cytosol protein extraction serving as a control . IP products are resolved on SDS-PAGE gel, silver stained, and specific bands are cut out for MS identification . This approach allows for unbiased identification of the antibody's target.

Recombinant expression validation provides additional confirmation. Candidate targets identified by MS are cloned into expression vectors (such as EGFP-tagged constructs) and validated using flow cytometry and immunofluorescence assays under non-permeable conditions . This approach confirms that the antibody specifically recognizes the identified protein when expressed in a controlled system.

Flow cytometry under non-permeable conditions is particularly important for validating antibodies intended for therapeutic use. Cells are fixed in paraformaldehyde but not permeabilized, ensuring that only cell surface binding is detected . Appropriate controls, including no primary antibody and isotype control antibody samples, are essential for confirming specificity .

For comprehensive validation, immunohistochemistry (IHC) on tissue microarrays containing both positive and negative control tissues provides additional evidence of specificity and reveals expression patterns across different tissue types . The staining pattern should be consistent with the expected subcellular localization of MUC18.

How can MUC18 expression be accurately detected in melanoma samples?

Accurate detection of MUC18 expression in melanoma samples is essential for patient stratification and evaluating the potential efficacy of MUC18-targeted therapies. Immunohistochemistry (IHC) on formalin-fixed, paraffin-embedded (FFPE) tissue sections is the most widely used approach for this purpose.

For IHC analysis, tissue processing typically involves deparaffinization and rehydration of sections, followed by antigen retrieval using appropriate buffers (e.g., DAKO antigen retrieval buffer) . Sections are then incubated with primary anti-MUC18 antibody (such as GeneTex cat# GTX01919) followed by horseradish peroxidase (HRP)–labeled secondary antibody . Detection is achieved using DAB staining buffer, with hematoxylin counterstain to visualize tissue architecture .

Quantification of MUC18 expression is commonly performed using the Histochemical scoring system (H-score) . This semi-quantitative approach considers both staining intensity and percentage of positive cells, with expression grade typically classified as:

• 0 (H-score 0–9)

• 1+ (H-score 10–99)

• 2+ (H-score 100–199)

• 3+ (H-score 200–300)

For research applications requiring more detailed analysis, multiplexed immunofluorescence can be used to simultaneously detect MUC18 along with other markers, such as vascular markers to distinguish between tumor cell and vascular expression. Flow cytometry provides another quantitative approach for assessing MUC18 expression in fresh tumor samples or cell lines, offering the advantage of measuring expression levels on a per-cell basis.

What are the key experimental models for testing MUC18 antibodies?

Selecting appropriate experimental models is crucial for evaluating the efficacy and safety of MUC18-targeting antibodies. Based on current research practices, the following models are commonly employed:

Cell line xenograft models utilize established metastatic melanoma lines that express high levels of MUC18, such as A375SM and WM2664 . These cells are typically injected subcutaneously into immunodeficient mice (e.g., nude mice), with antibody treatment beginning when tumors reach approximately 100 mm³ . Standard protocols involve weekly administration of antibody (e.g., 100 μg, i.p. for 5 weeks) . While these models are reproducible and well-characterized, they lack immune components and may not fully recapitulate the heterogeneity of clinical tumors.

Patient-derived xenograft (PDX) models involve direct implantation of patient tumor fragments into immunodeficient mice. This approach better preserves tumor heterogeneity and architecture, potentially providing better prediction of clinical response . PDX models are particularly valuable for testing across different melanoma subtypes (cutaneous, mucosal, acral, uveal) .

For assessing effects on metastasis, experimental metastasis models involving tail vein injection of melanoma cells can evaluate the impact on lung metastasis formation . This approach specifically assesses effects on later stages of the metastatic process, though it bypasses early steps such as local invasion and intravasation.

To evaluate anti-angiogenic effects, models that allow assessment of tumor vasculature are essential. These include the analysis of tumor vessel density in xenograft models using immunohistochemistry for endothelial markers, as well as in vitro vessel formation assays using human umbilical vein endothelial cells (HUVECs) .

How do antibody-drug conjugates targeting MUC18 compare to conventional antibodies?

Antibody-drug conjugates (ADCs) targeting MUC18 represent a significant advancement over conventional "naked" anti-MUC18 antibodies in terms of therapeutic potential. While conventional antibodies like ABX-MA1 primarily function through blocking MUC18-mediated processes, ADCs combine this targeting specificity with potent cytotoxic payloads.

Conventional anti-MUC18 antibodies such as ABX-MA1 have been shown to inhibit tumor growth and metastasis in vivo despite having no effect on melanoma cell proliferation rate in vitro . Their mechanism relies on disrupting cellular interactions, including homotypic interactions between melanoma cells and heterotypic interactions with vascular endothelial cells . They also inhibit matrix metalloproteinase 2 activity and interfere with angiogenesis .

In contrast, MUC18-directed ADCs like AMT-253 add direct cytotoxicity to these mechanisms. AMT-253, which uses the topoisomerase I inhibitor exatecan as its payload, exhibits MUC18-specific cytotoxicity through DNA damage and apoptosis . It also demonstrates a strong bystander killing effect, where released payload molecules can affect neighboring tumor cells even if they express lower levels of MUC18 .

The choice of payload significantly impacts ADC efficacy. AMT-253, with its exatecan payload and self-immolative T moiety, demonstrated a higher therapeutic index compared to a microtubule inhibitor-based counterpart (using vc-MMAE) . This highlights the importance of optimizing the payload for the specific target and tumor type.

Notably, AMT-253 has shown potent antitumor activities against both melanoma cell line xenografts and patient-derived xenograft models representative of various melanoma subtypes . Its efficacy extends beyond melanoma to a wide range of MUC18-expressing solid tumors, suggesting broader therapeutic potential .

What mechanisms contribute to the anti-tumor effects of MUC18 antibodies?

The anti-tumor effects of MUC18 antibodies are mediated through multiple mechanisms acting on both tumor cells and the tumor microenvironment. Understanding these mechanisms is crucial for optimizing therapeutic approaches and predicting clinical efficacy.

Disruption of cell adhesion is a primary mechanism. MUC18 antibodies interfere with both homotypic (melanoma cell-to-cell) and heterotypic (melanoma cell-to-endothelial cell) interactions . This disruption has been demonstrated experimentally, where ABX-MA1 disrupted spheroid formation by melanoma cells expressing MUC18 and inhibited the ability of these cells to attach to human vascular endothelial cells (HUVECs) .

Inhibition of matrix metalloproteinase activity represents another key mechanism. ABX-MA1 treatment of melanoma cells in vitro significantly inhibits the promoter and collagenase activity of matrix metalloproteinase 2, resulting in decreased invasion through Matrigel-coated filters . Decreased expression of matrix metalloproteinase 2 was also observed in implanted tumors in vivo following antibody treatment .

Anti-angiogenic effects contribute significantly to tumor suppression. Since MUC18 is expressed on endothelial cells as well as tumor cells, anti-MUC18 antibodies can directly disrupt angiogenesis . ABX-MA1 has been shown to interfere with tube-like formation by HUVECs in an in vitro vessel formation assay . Additionally, a mouse MUC18-specific antibody-T1000-exatecan conjugate demonstrated antitumor efficacy in human melanoma xenografts by targeting the mouse tumor vasculature .

When developed as ADCs, additional mechanisms come into play. AMT-253 exhibited MUC18-specific cytotoxicity through DNA damage (as evidenced by phospho-H2A.X induction) and apoptosis (via caspase activation) . The ADC also demonstrated a strong bystander killing effect, enabling it to affect nearby tumor cells even if they express lower levels of the target .

How does the dual-targeting of tumor cells and vasculature enhance MUC18 antibody efficacy?

MUC18 presents a unique opportunity for dual-compartment targeting as it is expressed both on tumor cells and tumor-infiltrating blood vessels across major melanoma subtypes . This dual-targeting capability potentially offers several therapeutic advantages over approaches targeting either compartment alone.

When targeting MUC18 on tumor cells, antibodies disrupt homotypic adhesion between tumor cells, inhibit MMP2 activation and invasive capacity, and, in the case of ADCs, deliver cytotoxic payloads directly to the tumor cells . These effects primarily impact the tumor cells themselves, reducing their growth and metastatic potential.

Simultaneously, the same antibodies can target MUC18 on tumor vasculature, disrupting endothelial cell function and angiogenesis . This vascular targeting affects the tumor indirectly by compromising its blood supply, potentially leading to reduced perfusion, nutrient delivery, and increased hypoxia in the tumor microenvironment.

The value of this dual-targeting approach has been demonstrated experimentally. A mouse MUC18-specific antibody-T1000-exatecan conjugate showed antitumor efficacy in human melanoma xenografts by targeting the mouse tumor vasculature . More compellingly, combination therapy of the tumor-targeting ADC AMT-253 with an antiangiogenic agent generated higher efficacy than either agent alone in a mucosal melanoma model . This synergistic effect suggests the value of simultaneously targeting both compartments.

This dual-compartment targeting capability may explain the broad efficacy of anti-MUC18 therapies across different melanoma subtypes . It may be particularly valuable in addressing tumor heterogeneity and reducing the likelihood of treatment resistance that often emerges with single-target approaches.

What strategies are being explored to enhance MUC18 antibody efficacy and overcome resistance?

Despite promising results with MUC18-targeting approaches, enhancing efficacy and overcoming potential resistance mechanisms remains an important research focus. Several strategic approaches are being explored:

Optimizing antibody-drug conjugate design represents a major avenue for improvement. This includes careful selection of payload type based on the specific characteristics of the target and tumor type. For instance, AMT-253, a MUC18-directed ADC based on topoisomerase I inhibitor exatecan with a self-immolative T moiety, demonstrated a higher therapeutic index compared to a microtubule inhibitor-based counterpart . This suggests that payload selection significantly impacts both efficacy and safety.

Combination therapy approaches are being investigated to enhance efficacy. Notably, combination therapy of AMT-253 with an antiangiogenic agent generated higher efficacy than single-agent therapy in a mucosal melanoma model . This suggests potential synergy between direct tumor cell targeting and further compromising tumor vasculature beyond what is achieved by the MUC18-ADC alone.

Addressing target heterogeneity is another important consideration. Since MUC18 expression may vary within tumors and between patients, approaches that can maintain efficacy despite this heterogeneity are valuable. The bystander killing effect observed with AMT-253 may help address this challenge by enabling the ADC to affect nearby tumor cells even if they express lower levels of MUC18 .

Expanding beyond melanoma, research has shown that MUC18-targeting ADCs like AMT-253 demonstrated efficacy in a wide range of MUC18-expressing solid tumors beyond melanoma . This suggests potential broader applications and the value of investigating MUC18 targeting across multiple cancer types.

Engineering improved antibodies with enhanced tumor penetration, reduced immunogenicity, and optimized binding properties represents another avenue for improving efficacy. This may involve antibody fragment approaches or novel antibody formats that maintain target binding while improving pharmacokinetic properties.

How can researchers optimize the pharmacokinetics of MUC18 antibodies?

Optimizing the pharmacokinetics (PK) of MUC18 antibodies is crucial for maximizing their therapeutic efficacy while minimizing potential toxicities. Several methodological approaches can be employed to achieve this goal:

The choice of linker technology significantly impacts pharmacokinetics. For AMT-253, a self-immolative T moiety was used with the topoisomerase I inhibitor exatecan . The linker design influences stability in circulation, selective release in the tumor microenvironment, and the potential for bystander effects. Linker optimization involves in vitro stability testing in various physiological conditions followed by in vivo PK assessment.

Species-specific testing provides important insights into clinical translatability. AMT-253 demonstrated favorable pharmacokinetics and tolerability in monkeys, suggesting good translational potential to humans . Conducting PK studies in non-human primates is particularly valuable for antibody therapeutics due to greater similarity in target distribution and antibody processing compared to rodent models.

Antibody engineering approaches can further optimize PK properties. This may include Fc engineering to enhance or reduce FcRn binding (affecting half-life), modifying glycosylation patterns, or creating novel antibody formats with improved tumor penetration characteristics. Each modification requires comparative PK assessment to evaluate its impact.

Toxicology studies should accompany PK investigations to establish the therapeutic window. For MUC18 antibodies, potential on-target off-tumor effects related to MUC18 expression in normal tissues, particularly endothelial cells, require careful evaluation. AMT-253 demonstrated a favorable tolerability profile, with acceptable endotoxin levels (less than 0.05 EU/mg) , an important consideration for therapeutic antibodies.

What techniques are used to develop the melanoma cell surface-binding antibody atlas?

The development of a comprehensive melanoma cell surface-binding antibody atlas represents a powerful approach for identifying novel therapeutic targets. The proteome-scale antibody array platform (PETAL) has been successfully applied to build such an atlas, leading to the identification of melanoma-associated cell surface antigens including MUC18 . The methodology involves several key steps:

The first stage involves screening a diverse panel of melanoma cell lines with a proteome-scale monoclonal antibody array. This approach is particularly valuable because cell surface membrane proteins are often under-represented by current genomics or proteomics approaches for target discovery . By directly screening for cell surface binding, PETAL overcomes this limitation and identifies antibodies that recognize accessible epitopes.

Flow cytometry serves as the primary screening tool, allowing rapid quantification of antibody binding to live cells. Positive hits are defined by antibodies showing significant binding compared to isotype controls, with different melanoma cell lines potentially yielding distinct binding profiles . This approach allows identification of antibodies that bind preferentially to melanoma cells compared to normal melanocytes.

Target identification for promising antibodies is achieved through immunoprecipitation (IP) followed by mass spectrometry (MS) . The cellular target of pAb253, for example, was identified using membrane extraction prepared from the FACS-positive cell line, with cytosol protein extraction of the same cell line used as control . IP products are resolved on SDS-PAGE gel, silver stained, and specific bands cut out for MS identification.

Validation of identified targets involves cloning candidates with predicted membrane localization into expression vectors (such as EGFP expression vectors) and confirming antibody binding using flow cytometry and immunofluorescence assays under non-permeable conditions . This step confirms that the antibody specifically recognizes the identified protein.

Clinical relevance assessment follows validation, evaluating target expression across patient samples using tissue microarrays (TMAs) and immunohistochemistry . This allows quantification of expression levels across different melanoma subtypes and correlation with clinical parameters. For MUC18, this approach revealed strong overexpression in the majority of patients across all subtypes of melanoma, with predominated cell surface location .

What protocols are used to evaluate the cytotoxicity of MUC18 antibody-drug conjugates?

Evaluating the cytotoxicity of MUC18 antibody-drug conjugates (ADCs) requires rigorous and systematic testing using multiple complementary approaches. The following protocols are commonly employed:

For direct ADC cytotoxicity assays, cells are typically seeded on 96-well plates at 2–5,000 cells/well depending on the growth rate . The next day, control IgG-conjugates or the test ADC (e.g., AMT-253) are serially diluted at appropriate concentrations and added to the wells . Cells are incubated at 37°C for 5 days, after which cell viability is assessed using the CCK-8 (Cell Counting Kit-8) assay . Cell viability is calculated as the percentage of control wells with only tumor cells.

For high-throughput screening of multiple antibody candidates, an indirect cytotoxicity assay strategy may be employed. This involves using a toxin-conjugated anti-mouse IgG antibody as a surrogate for directly conjugating mAbs . Cancer cells are plated and cultured overnight, then treated with serial dilutions of the test antibody together with the toxin-conjugated secondary antibody for 72 hours . This approach allows efficient screening of numerous antibody candidates.

To distinguish between on-target and off-target effects, specificity controls are essential. These typically include isotype-matched control antibodies conjugated to the same payload, unconjugated parent antibody, and free payload . Target-negative cell lines should show minimal sensitivity to the ADC while remaining sensitive to free payload.

For evaluating mechanism of action, additional assays are performed. Phosphohistone H2A.X (Ser139) staining can detect DNA damage, while cleaved caspase-3 and cleaved caspase-8 staining can assess apoptosis induction . These markers can be evaluated both in vitro using immunoblotting or immunofluorescence and in vivo in treated tumor samples using immunohistochemistry.

To assess bystander killing effects, co-culture systems can be established with MUC18-positive and MUC18-negative cells. Cell viability is then assessed in both populations following ADC treatment. AMT-253 has been documented to exhibit a strong bystander killing effect, which contributes to its potent antitumor activities against melanoma models .

For in vivo efficacy testing, tumor-bearing mice are treated with the ADC according to specified dosing schedules. Tumor growth is monitored over time, and at study endpoint, tumors are harvested for pharmacodynamic marker assessment . This approach provides the most physiologically relevant assessment of ADC efficacy.

How are MUC18 antibodies evaluated for their effects on tumor vasculature?

Given MUC18's expression in tumor-infiltrating blood vessels, evaluating the anti-angiogenic effects of MUC18 antibodies is crucial for understanding their complete mechanism of action. Several complementary approaches are used:

The endothelial tube formation assay provides a direct assessment of anti-angiogenic potential in vitro. Human umbilical vein endothelial cells (HUVECs), which express MUC18, are cultured on Matrigel to form tube-like structures resembling capillaries . When treated with anti-MUC18 antibodies such as ABX-MA1, disruption of this tube formation can be observed and quantified . This assay directly demonstrates the ability of MUC18 antibodies to interfere with a key step in angiogenesis.

Analysis of tumor vasculature in xenograft models provides in vivo evidence of anti-angiogenic effects. Following treatment with MUC18 antibodies, tumors are harvested and analyzed by immunohistochemistry for endothelial markers such as CD31 . Vessel density, diameter, and morphology can be quantified to assess the impact on tumor vasculature. Functional assessment of vessel perfusion can be achieved using injected dyes or labeled lectins.

The dual-targeting potential of MUC18 antibodies can be specifically evaluated using species-selective approaches. For example, a mouse MUC18-specific antibody-T1000-exatecan conjugate has been shown to inhibit tumor growth in human melanoma xenografts by targeting the mouse tumor vasculature . This elegant approach distinguishes between effects on human tumor cells and mouse-derived tumor vasculature.

Combination therapy studies with established anti-angiogenic agents provide insights into complementary or synergistic effects. Notably, combination therapy of AMT-253 with an antiangiogenic agent generated higher efficacy than single-agent therapy in a mucosal melanoma model . This suggests potential synergy between direct tumor cell targeting and further compromising tumor vasculature.

Molecular analyses of angiogenic factors and signaling pathways complement functional assays. These may include assessment of vascular endothelial growth factor (VEGF) expression, hypoxia-inducible factor (HIF) activation, and endothelial cell activation markers. Such analyses help elucidate the molecular mechanisms underlying the observed anti-angiogenic effects.

What methodologies are used to assess MUC18 antibody internalization kinetics?

Evaluating the internalization kinetics of MUC18 antibodies is critical, particularly for the development of antibody-drug conjugates where payload delivery depends on efficient internalization. Several methodological approaches are employed:

Flow cytometry-based internalization assays provide quantitative assessment of antibody uptake over time. Cells are typically incubated with fluorescently-labeled MUC18 antibody at 4°C to allow surface binding, then shifted to 37°C to initiate internalization . Surface-bound antibody can be distinguished from internalized antibody using acid wash to strip surface antibody or by using pH-sensitive fluorophores that change intensity upon internalization into acidic endosomal compartments.

Confocal microscopy offers spatial resolution to track antibody trafficking within cells. Cells are treated with labeled antibody as above, then fixed at various time points and counterstained with markers for different cellular compartments such as early endosomes (EEA1), late endosomes (Rab7), and lysosomes (LAMP1) . Co-localization analysis reveals the intracellular fate of the internalized antibody, which is particularly important for ADCs with different linker chemistries requiring specific cellular environments for payload release.

For antibody-drug conjugates, correlating internalization kinetics with cytotoxicity provides functional relevance. Cells can be treated with the ADC for varying durations, washed to remove unbound ADC, and then assessed for viability after continued culture . The minimum exposure time required for maximal cytotoxicity provides insights into the efficiency of internalization and payload release.

Biochemical approaches offer complementary quantitative data. These may involve surface biotinylation of the antibody, followed by incubation with cells at 4°C and then shifting to 37°C to initiate internalization. At various time points, remaining surface biotin can be stripped using a membrane-impermeable reducing agent, allowing specific quantification of internalized antibody by Western blot or ELISA .

Comparing internalization across different cell lines expressing varying levels of MUC18 can reveal the relationship between receptor density and internalization efficiency. This is particularly important for predicting ADC efficacy across heterogeneous tumors with variable target expression.

How is the bystander killing effect of MUC18 antibody-drug conjugates measured?

The bystander killing effect, where released payload molecules diffuse to and kill neighboring cells, is an important mechanism for ADC efficacy, particularly in tumors with heterogeneous target expression. For MUC18-targeted ADCs like AMT-253, which has demonstrated a strong bystander killing effect, several methodological approaches can be used to assess this phenomenon:

Co-culture systems provide the most direct assessment of bystander killing. MUC18-positive and MUC18-negative cells (either naturally occurring or generated via CRISPR knockout) are co-cultured at defined ratios . The cells can be differentially labeled with fluorescent proteins or dyes to distinguish them during analysis. After treatment with the MUC18-ADC, the viability of both populations is assessed using flow cytometry. The degree of killing observed in the MUC18-negative population relative to the MUC18-positive cells indicates the strength of the bystander effect.

Three-dimensional spheroid models better recapitulate solid tumor architecture and diffusion barriers. Mixed spheroids containing both MUC18-positive and MUC18-negative cells are treated with the ADC, and cell death patterns are analyzed using confocal microscopy with viability dyes . This approach allows assessment of spatial distribution of cell death relative to MUC18 expression patterns, providing insights into payload diffusion within a three-dimensional environment.

For mechanistic understanding, direct measurement of intracellular payload accumulation in bystander cells can be performed. This typically involves treating MUC18-positive cells with the ADC, then collecting the culture medium after appropriate incubation to allow for payload release . This conditioned medium is then applied to MUC18-negative cells, and intracellular payload accumulation is measured using methods appropriate for the specific payload (e.g., fluorescence microscopy for fluorescent payloads or mass spectrometry).

Comparing bystander effects between different linker-payload combinations provides valuable insights for ADC optimization. For example, membrane-permeable payloads like exatecan (used in AMT-253) typically demonstrate stronger bystander effects than charged or membrane-impermeable payloads . Systematic comparison can guide linker-payload selection for tumors with heterogeneous target expression.

In vivo assessment provides the most physiologically relevant evaluation. This can involve establishing mixed tumors with defined ratios of MUC18-positive and MUC18-negative cells (differentially labeled), treating with the MUC18-ADC, and analyzing changes in the proportion of each population over time . Immunohistochemistry can also assess spatial patterns of cell death relative to MUC18 expression within tumors.