Unexpected hematologic effects of biotherapeutics in nonclinical species and in humans

[Mobilization and conditioning protocols actualization for autologous stem cell transplantation for autoimmune diseases: Guidelines from MATHEC-SFGM-TC]

The Francophone Society of Bone Marrow Transplantation and Cellular Therapy (SFGM-TC) organized the 13th workshop on hematopoietic stem cell transplantation clinical practices harmonization procedures in September 2022 in Lille, France. The aim of this workshop is to update the mobilization and conditioning protocols for autologous hematopoietic stem cell transplantation for autoimmune diseases, and to specify contraindications for transplant, conditioning regimen selection, immunosuppressive treatment discontinuation before mobilization and disease-specific surveillance.

Research-Relevant Clinical Pathology Resources: Emphasis on Mice, Rats, Rabbits, Dogs, Minipigs, and Non-Human Primates

Clinical pathology testing for investigative or biomedical research and for preclinical toxicity and safety assessment in laboratory animals is a distinct specialty requiring an understanding of species specific and other influential variables on results and interpretation. This review of clinical pathology principles and testing recommendations in laboratory animal species aims to provide a useful resource for researchers, veterinary specialists, toxicologists, and clinical or anatomic pathologists.

Adalimumab-Induced Thrombocytopenia in a Patient With Hidradenitis Suppurativa

Adalimumab-induced thrombocytopenia is a rarely occurring condition that may present with hemorrhagic manifestations. This report describes a case of a patient who presented with severe, symptomatic thrombocytopenia while on adalimumab for the treatment of hidradenitis suppurativa. The patient responded to treatment with steroids, intravenous immunoglobulin (IVIG), and platelet transfusion, in addition to discontinuation of adalimumab.

Nonclinical Safety Assessment of AMG 553, an Investigational Chimeric Antigen Receptor T-Cell Therapy for the Treatment of Acute Myeloid Leukemia

Feline McDonough Sarcoma-like tyrosine kinase 3 (FLT3), a tyrosine-protein kinase involved in hematopoiesis, is detectable on the cell surface of approximately 80% of leukemia isolates from adult patients with acute myeloid leukemia (AML). AMG 553 is an investigational chimeric antigen receptor (CAR) T-cell immunotherapy for the treatment of AML. FLT3 expression analysis and in vitro and in vivo studies were leveraged to evaluate the nonclinical safety of AMG 553. Cynomolgus monkeys administered autologous anti-FLT3 CAR T cells demonstrated no evidence of CAR T-cell-mediated toxicity, expansion, or persistence, likely due to restricted cell surface FLT3 protein expression in healthy animals. This highlights the limited value of such in vivo studies for safety assessment of the CAR T-cell modality when directed against a target with restricted expression. To complement these studies and directly evaluate the potential toxicities of eliciting T-cell-mediated cytotoxicity against cells with surface expression of FLT3 protein in vivo, data from cynomolgus monkey toxicology studies with 2 bispecific T-cell engager molecules targeting FLT3 were leveraged; findings were consistent with the targeted killing of bone marrow cells expressing cell surface FLT3. Potential AMG 553-induced cytotoxicity was assessed against a wide range of normal human primary cells and cell lines; cytotoxicity was observed against FLT3-positive AML cell lines and a percentage of primary bone marrow CD34+ cells. In conclusion, the nonclinical safety data suggest that AMG 553 can target FLT3 protein on AML cells, whereas only affecting a percentage of normal hematopoietic stem and progenitor cells, supporting clinical development.

A Pilot Study on the Safety of a Novel Antioxidant Nanoparticle Delivery System and Its Indirect Effects on Cytokine Levels in Four Dogs

Acute spinal cord injury consists of a primary, traumatic event followed by a cascade of secondary events resulting in ongoing cell damage and death. There is great interest in prevention of these secondary effects to reduce permanent long-term neurologic deficits. One such target includes reactive oxygen species released following injury, which can be enzymatically converted into less harmful molecules by superoxide dismutase and catalase. Canine intervertebral disc herniation has been suggested as a naturally occurring model for acute spinal cord injury and its secondary effects in people. The aims of this study were to test the safety of a novel antioxidant delivery system in four healthy dogs and to indirectly test effect of delivery via cytokine measurement. All dogs experienced adverse events to some degree, with two experiencing adverse events considered to be severe. The clinical signs, including combinations of bradycardia, hypotension, hypersalivation, pale gums, and involuntary urination, were consistent with complement activation-related pseudoallergy (CARPA). CARPA is a well-known phenomenon that has been reported to occur with nanoparticle-based drug delivery, among other documented causes. Two dogs also had mild to moderate changes in their blood cell count and chemistry, including elevated alanine transferase, and thrombocytopenia, which both returned to normal by day 7 post-administration. Cytokine levels trended downwards over the first 3 days, but many were elevated at measurement on day 7. Intradermal testing suggested catalase as a potential cause for reactions. No long-term clinical signs were observed, and necropsy results revealed no concerning pathology. Additional evaluation of this product, including further characterization of reactions to catalase containing components, dose-escalation, and desensitization should be performed before evaluation in clinically affected dogs.

Optimization of clinical dosing schedule to manage neutropenia: learnings from semi-mechanistic modeling simulation approach

Neutropenia is a common side-effect of oncology drugs. We aimed to study the impact of exposure and dosing schedule on neutropenia to guide selection of dosing schedules that minimize neutropenia potential while maintaining the desired minimum concentration (C_min) required for target engagement. Dose, frequency and PK parameters were chosen for five hypothetical drugs of various half-lives to (1) achieve same exposure with continuous dosing and evaluate impact of 4 intermittent dosing schedules; and (2) achieve same nadir for continuous and intermittent dosing and evaluate impact on % time above C_min, a surrogate assumed to indicate target engagement. Absolute neutrophil count (ANC) profiles were simulated using Friberg model, a widely used semi-mechanistic myelosuppression model, assuming drug concentration directly reduce the proliferation rate of stem cells and progenitor cells in proliferation compartment. The correlations between different PK measures and neutropenia metrics were explored. In (1), when the same daily dose was used, intermittent schedules offered better management of ANC nadir. The reduced average drug exposure with intermittent dosing led to lower% time above C_min. In (2), when the dose was adjusted to achieve the same nadir, drugs with moderate half-life (8-48 h) showed similar % time above C_min regardless of schedule, while continuous dosing was better for a short half-life (4 h). Area under the concentration curve (AUC) was highly correlated with neutropenia. In summary, continuous dosing, with the dose selected correctly, is most effective to maintain % time above C_min while providing similar tolerability as intermittent dosing with a higher dose. But dose interruptions could be required to manage individual toxicities. Intermittent schedules, on the other hand, allow recovery of ANC, enabling more orderly schedules.

Understanding Platelets in Infectious and Allergic Lung Diseases

Emerging evidence suggests that platelets, cytoplasmic fragments derived from megakaryocytes, can no longer be considered just as mediators in hemostasis and coagulation processes, but as key modulators of immunity. Platelets have received increasing attention as the emergence of new methodologies has allowed the characterization of their components and functions in the immune continuum. Platelet activation in infectious and allergic lung diseases has been well documented and associated with bacterial infections reproduced in several animal models of pulmonary bacterial infections. Direct interactions between platelets and bacteria have been associated with increased pulmonary platelet accumulation, whereas bacterial-derived toxins have also been reported to modulate platelet function. Recently, platelets have been found extravascular in the lungs of patients with asthma, and in animal models of allergic lung inflammation. Their ability to interact with immune and endothelial cells and secrete immune mediators makes them one attractive target for biomarker identification that will help characterize their contribution to lung diseases. Here, we present an original review of the last advances in the platelet field with a focus on the contribution of platelets to respiratory infections and allergic-mediated diseases.

Pros and cons of the immunogenicity of monoclonal antibodies in cancer treatment: a lesson from autoimmune diseases

The aim of this review is to report the current evidence on immunogenicity of monoclonal antibodies (moAbs) used in cancer compared with autoimmune diseases, focusing on local microenvironment. English abstracts were identified in Medline and www.clinicaltrials.gov . A total of 82 papers were selected. The percentage of immunogenicity of moAbs used for cancer therapy, evaluated as the serum concentration of antidrug antibodies, is significantly lower than that of moAbs used for the treatment of autoimmune diseases. This condition may rely on a different immunologic background characterized by a hyperactivation of immune cells in autoimmune diseases. The formation of complexes between antidrug antibodies and non-neutralizing moAbs bound to neoplastic antigens may allow more efficient elimination of cancer cells, but additional studies are needed.

In Vitro Monocyte/Macrophage Phagocytosis Assay for the Prediction of Drug-Induced Thrombocytopenia

Phagocytosis of platelets by monocytes and macrophages is a primary mechanism of platelet clearance in vivo and has been increasingly implicated in playing an important role in thrombocytopenia mediated by monoclonal antibodies intended for therapeutic purposes. In the present article, we describe an in vitro flow cytometry assay to assess the effect of antibody-mediated platelet phagocytosis by monocytes. Freshly isolated platelets were labeled with a fluorescent probe, 5-chloromethylfluorescein diacetate (CMFDA) and then co-cultured with isolated peripheral blood mononuclear cells (PBMCs) from the same donor in the presence of increasing concentrations of a monoclonal antibody drug. After incubation, an increase in CMFDA fluorescence intensity of CD14 positive monocytes was evaluated by flow cytometry as an assessment for drug-mediated platelet phagocytosis by monocytes. The assay has been evaluated using both human and cynomolgus monkey cells for the prediction of drug-induced thrombocytopenia. © 2019 by John Wiley & Sons, Inc.

Histopathological Findings in Cynomolgus Macaques ( Macaca fascicularis) Consistent with Secondary Immunological Reaction to Biotherapeutics with an Emphasis on the CNS and Eye

Biotherapeutics are pharmaceutical products derived from or synthesized by biological systems. Such molecules carry the potential for immunogenicity which may lead to adverse immune responses. The cynomolgus macaque ( Macaca fascicularis) is the species of choice in nonclinical safety assessment of biotherapeutics. The main aim of this study was to confirm whether mononuclear cell infiltrates at specific locations represent a generic effect of biotherapeutics, and therefore the result of their immunogenicity. Following a review of microscopic findings in studies conducted over a 10-year period at one test facility, 15% of biotherapeutics were reported to have such findings. The most commonly affected site was the choroid plexus and less frequently the meninges and ciliary body. The reporting of such findings as test article-related becomes more subjective as the severity and incidence decreases. To assess the accuracy of such associations, a mathematical approach was employed to determine the probability of obtaining the observed results by chance. There was good agreement between this approach and the original findings. In addition to an increased number and size of mononuclear cell infiltrates in the brain, biotherapeutic administration was strongly associated with the presence of plasma cells and eosinophils.

Proof of Concept for an Automated Image Analysis Method to Quantify Rat Bone Marrow Hematopoietic Lineages on H&E Sections

The bone marrow is an important site for assessment of the hematopoietic toxicity of new drug candidates. Here, we extended our previous work, where we developed a computer algorithm to automatically quantitate overall bone marrow cell density by analyzing digitized images of standard hematoxylin and eosin (H&E) slides of rat bone marrow and further evaluated the capability to quantify myeloid: erythroid + lymphoid (M:EL) ratio and megakaryocyte cell density. We tested the algorithm in a toxicity study, where rats were dosed with two molecules known to affect bone marrow composition, monomethyl auristatin E, and a Bcl-xL inhibitor. The image analysis method detected significant changes in M:EL and megakaryocyte number that were either not found or semiquantitatively described by manual microscopic observation of the same slides. The image analysis results were consistent with other more established but time-consuming methods that measure changes in bone marrow cell composition: smear cytology, flow cytometry, and microscopic assessment. Our work demonstrates the feasibility of a rapid and more quantitative assessment of changes in bone marrow cell lineage composition using a computer algorithm compared to microscopic examination of H&E-stained bone marrow sections.

Cetuximab or nimotuzumab plus intensity-modulated radiotherapy versus cisplatin plus intensity-modulated radiotherapy for stage II-IVb nasopharyngeal carcinoma

To compare intensity-modulated radiotherapy (IMRT) with cisplatin (CDDP) versus cetuximab (CTX) and nimotuzumab (NTZ) for Stage II-IVb Nasopharyngeal Carcinoma (NPC). A total of 1,837 patients with stage II-IV_b NPC who received IMRT plus CTX or NTZ, or CDDP between January 2009 and December 2013 were included in the current analysis. Using propensity scores to adjust for potential prognostic factors, a well-balanced cohort of 715 patients was created by matching each patient who underwent IMRT plus concomitant NTZ/CTX with four patients who underwent IMRT plus concomitant CDDP (1:4). Efficacy and safety were compared between the CTX/NTZ and CDDP groups of this well-balanced cohort. Furthermore, we conducted multivariate analysis and subgroup analysis based on all the 1,837 eligible cases. There was no significant difference between CTX/NTZ group and CDDP group in terms of DFS (3-year, 86.7% vs. 86.2%, p > 0.05), LRRFS (96.2% vs. 96.3%, p > 0.05), DMFS (91.1% vs. 92.3%, p > 0.05) and OS (91.7% vs. 91.9%, p > 0.05). Subgroup analysis demonstrated a significant interaction effect between patients with IMRT plus CTX/NTZ and N3 node stage on LRRFS with the highest risk of loco-regional relapse (HR 8.85, p = 0.001). Significantly increased hematologic toxicities, gastrointestinal reactions were observed in the CDDP group (p < 0.05). Patients of 3.4-4.7% experienced severe hematologic toxicities during the treatment with concomitant CTX and NTZ. Increased rate of CTX related-skin reaction and mucositis was observed in the CTX group. CTX/NTZ used concurrently with IMRT may be comparable to those of the standard CDDP-IMRT combination for maximizing survival for patients with stage II-IVb NPC.

Biomarkers for nonclinical infusion reactions in marketed biotherapeutics and considerations for study design

The observation of an infusion reaction (IR) in a nonclinical study can cause concern among investigators and regulators in the development of biotherapeutics. Biomarkers can be informative to determine whether the reactions are immune-mediated or test-article related and if there is a potential risk to human subjects. IRs encompass a broad range of adverse events with a variety of triggers; the focus of this paper is IRs due to cytokine release syndrome or immune complex formation and the associated biomarkers. Such reactions generally do not preclude clinical development or marketing approval, because it is widely accepted that immune-mediated reactions in nonclinical species are not predictive of human outcomes. Several US approved products (from 2004 to 2016) have documented IRs in nonclinical species. This review article discusses recent examples, the biomarkers evaluated, and implications for study design and conduct.

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Approved biotherapeutics have produced nonclinical infusion reactions (IRs).

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Nonclinical IRs after a first dose are associated with cytokine release.

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Nonclinical IRs after several doses are associated with ADA.

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ADA-mediated IRs may result in immune complex tissue deposition.

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Diagnosing nonclinical IRs requires a weight-of-evidence approach using biomarkers.

Engineering an IgG Scaffold Lacking Effector Function with Optimized Developability

IgG isotypes can differentially bind to Fcγ receptors and complement, making the selection of which isotype to pursue for development of a particular therapeutic antibody important in determining the safety and efficacy of the drug. IgG2 and IgG4 isotypes have significantly lower binding affinity to Fcγ receptors. Recent evidence suggests that the IgG2 isotype is not completely devoid of effector function, whereas the IgG4 isotype can undergo in vivo Fab arm exchange leading to bispecific antibody and off-target effects. Here an attempt was made to engineer an IgG1-based scaffold lacking effector function but with stability equivalent to that of the parent IgG1. Care was taken to ensure that both stability and lack of effector function was achieved with a minimum number of mutations. Among the Asn²⁹⁷ mutants that result in lack of glycosylation and thus loss of effector function, we demonstrate that the N297G variant has better stability and developability compared with the N297Q or N297A variants. To further improve the stability of N297G, we introduced a novel engineered disulfide bond at a solvent inaccessible location in the CH2 domain. The resulting scaffold has stability greater than or equivalent to that of the parental IgG1 scaffold. Extensive biophysical analyses and pharmacokinetic (PK) studies in mouse, rat, and monkey further confirmed the developability of this unique scaffold, and suggest that it could be used for all Fc containing therapeutics (e.g. antibodies, bispecific antibodies, and Fc fusions) requiring lack of effector function or elimination of binding to Fcγ receptors.

Biological Characterization of a Stable Effector Functionless (SEFL) Monoclonal Antibody Scaffold in Vitro

The stable effector functionLess (SEFL) antibody was designed as an IgG1 antibody with a constant region that lacks the ability to interact with Fcγ receptors. The engineering and stability and pharmacokinetic assessments of the SEFL scaffold is described in the accompanying article (Jacobsen, F. W., Stevenson, R., Li, C., Salimi-Moosavi, H., Liu, L., Wen, J., Luo, Q., Daris, K., Buck, L., Miller, S., Ho, S-Y., Wang, W., Chen, Q., Walker, K., Wypych, J., Narhi, L., and Gunasekaran, K. (2017) J. Biol. Chem 292). The biological properties of these SEFL antibodies were assessed in a variety of human and cynomolgus monkey in vitro assays. Binding of parent molecules and their SEFL variants to human and cynomolgus monkey FcγRs were evaluated using flow cytometry-based binding assays. The SEFL variants tested showed decreased binding affinity to human and cynomolgus FcγRs compared with the wild-type IgG1 antibody. In addition, SEFL variants demonstrated no antibody-dependent cell-mediated cytotoxicity in vitro against Daudi cells with cynomolgus monkey peripheral blood mononuclear cells, and had minimal complement-dependent cytotoxicity activity similar to that of the negative control IgG2 in a CD20⁺ human Raji lymphoma cell line. SEFL mutations eliminated off-target antibody-dependent monocyte phagocytosis of cynomolgus monkey platelets, and cynomolgus platelet activation in vitro These experiments demonstrate that the SEFL modifications successfully eliminated Fc-associated effector binding and functions.

Cetuximab and Radiotherapy Versus Cisplatin and Radiotherapy for Locally Advanced Head and Neck Cancer: A Randomized Phase II Trial

PURPOSE

No randomized trials have been conducted to directly compare radiotherapy (RT) with concomitant cisplatin (CDDP) versus concomitant cetuximab (CTX) as first-line treatment of locally advanced squamous cell carcinoma of the head and neck. In this randomized trial, we compared these two treatment regimens in terms of compliance, toxicity, and efficacy.

PATIENTS AND METHODS

Eligible patients were randomly assigned in a 1:1 ratio to receive either CDDP 40 mg/m(2) once per week or CTX 400 mg/m(2) as loading dose followed by CTX 250 mg/m(2) once per week concomitant to radical RT. For primary end points, compliance to treatment was defined as number of days of treatment discontinuation and drug dosage reduction. The acute toxicity rate was defined according to the National Cancer Institute Common Toxicity Criteria. Efficacy end points were local recurrence-free survival, metastasis-free survival, cancer-specific survival, and overall survival.

RESULTS

The study was discontinued early because of slow accrual after the enrollment of 70 patients. RT discontinuation for more than 10 days occurred in 13% of patients given CTX and 0% given CDDP (P = .05). Drug dosage reduction occurred in 34% given CTX and 53% given CDDP (difference not significant). Toxicity profiles differed between the two arms, with hematologic, renal, and GI toxicities more frequent in the CDDP arm, and cutaneous toxicity and the need for nutritional support more frequent in the CTX arm. Serious adverse events related to treatment, including four versus one toxic deaths, were higher in the CTX arm (19% v 3%, P = .044). Locoregional control, patterns of failure, and survivals were similar between the treatment arms.

CONCLUSION

CTX concomitant to RT lowered compliance and increased acute toxicity rates. Efficacy outcomes were similar in both arms. These results raise the issue of appropriately selecting patients with head and neck cancer who can benefit from CTX in combination with RT.

From defining antigens to new therapies in multiple sclerosis: honoring the contributions of Ruth Arnon and Michael Sela

Ruth Arnon and Michael Sela profoundly influenced the development of a model system to test new therapies in multiple sclerosis (MS). Their application of the animal model, known as experimental autoimmune encephalomyelitis (EAE), for the discovery of Copaxone, opened a new path for testing of drug candidates in MS. By measuring clinical, pathologic, and immunologic outcomes, the biological implications of new drugs could be elucidated. Using EAE they established the efficacy of Copaxone as a therapy for preventing and reducing paralysis and inflammation in the central nervous system without massive immune suppression. This had a huge impact on the field of drug discovery for MS. Much like the use of parabiosis to discover soluble factors associated with obesity, or the replica plating system to probe antibiotic resistance in bacteria, the pioneering research on Copaxone using the EAE model, paved the way for the discovery of other therapeutics in MS, including Natalizumab and Fingolimod. Future applications of this approach may well elucidate novel therapies for the neurodegenerative phase of multiple sclerosis associated with disease progression.

Severe thrombocytopenia in a patient with hepatitis C treated with eltrombopag from off-label drug use to on-label drug use: a case report

Introduction

Off-label drug use refers to drug use beyond the specifications authorized for marketing. Eltrombopag is a new thrombopoietin receptor agonist which was used successfully in this critical case of thrombocytopenia associated with hepatitis C infection before it became an approved drug for such cases.

Case presentation

A 56-year-old Kuwaiti woman with hepatitis C virus infection was treated with pegylated interferon α-2a and ribavirin, laboratory test results prior to therapy were within normal values. After 4 weeks of that treatment, she developed neutropenia and severe thrombocytopenia. Her hepatitis C virus treatment was stopped for many years until eltrombopag was used as an off-label drug with an episode of severe thrombocytopenia. Her platelets count returned to normal level when triple therapy for hepatitis C virus was used successfully.

Conclusions

An off-label drug should be used only when it is the best available drug, based on evidence from on-going multicenter trials. It could be life saving for some patients in critical situations. However, clinical use of eltrombopag later confirmed that it is a safe and effective drug for immune thrombocytopenic purpura or thrombocytopenia associated with hepatitis C virus infection.

Reversible bone marrow aplasia induced by pegylated interferon-α-2a therapy in a patient with primary myelofibrosis

Interferon has been widely used in the management of patients with hematological malignancies such as polycythemia vera, myelofibrosis, chronic myeloid leukemia and viral infections such as chronic hepatitis C. Hematological adverse effects such as cytopenias have been observed, particularly in patients who receive a combination of interferon-α-2a and ribavirin for hepatitis C. Mild myelosuppression can be seen with pegylated interferon; however, bone marrow aplasia in patients with myelofibrosis has not been reported. It is important to be aware of such a serious complication since persistent bone marrow aplasia can be fatal. We describe a case of pegylated interferon-induced reversible bone marrow aplasia in a patient with primary myelofibrosis.

Immunogenicity of therapeutic proteins: influence of aggregation

The elicitation of anti-drug antibodies (ADA) against biotherapeutics can have detrimental effects on drug safety, efficacy, and pharmacokinetics. The immunogenicity of biotherapeutics is, therefore, an important issue. There is evidence that protein aggregation can result in enhanced immunogenicity; however, the precise immunological and biochemical mechanisms responsible are poorly defined. In the context of biotherapeutic drug development and safety assessment, understanding the mechanisms underlying aggregate immunogenicity is of considerable interest. This review provides an overview of the phenomenon of protein aggregation, the production of unwanted aggregates during bioprocessing, and how the immune response to aggregated protein differs from that provoked by non-aggregated protein. Of particular interest is the nature of the interaction of aggregates with the immune system and how subsequent ADA responses are induced. Pathways considered here include 'classical' activation of the immune system involving antigen presenting cells and, alternatively, the breakdown of B-cell tolerance. Additionally, methods available to screen for aggregation and immunogenicity will be described. With an increased understanding of aggregation-enhanced immune responses, it may be possible to develop improved manufacturing and screening processes to avoid, or at least reduce, the problems associated with ADA.

Unexpected Thrombocytopenia and Anemia in Cynomolgus Monkeys Induced by a Therapeutic Human Monoclonal Antibody

Cynomolgus monkeys dosed with a therapeutic monoclonal antibody (mAbY.1) at ≥50 mg/kg had unexpected acute thrombocytopenia (nadir ∼3,000 platelets/µl), sometimes with decreases in red cell mass. Increased activated macrophages, mitotic figures, and erythrophagocytosis were observed in the spleen. Binding of mAbY.1 to cynomolgus peripheral blood cells could not be detected in vitro. mAbY.1 induced phagocytosis of platelets by peripheral blood monocytes from cynomolgus monkeys, but not from humans. mAbs sharing the same constant domain (Fc) sequences, but differing from mAbY.1 in their variable domains, bound competitively to and had similar biological activity against the intended target. None of these antibodies had hematologic liabilities in vitro or in vivo. Neither the F(ab’)2 portion of mAbY.1 nor the F(ab’)2 portion on an aglycosylated Fc (IgG1) framework caused phagocytosis of platelets in vitro. These data suggest that the hematologic effects of mAbY.1 in cynomolgus monkeys likely occurred through an off-target mechanism, shown to be driven by 1 to 3 amino acid differences in the light chain. The hematologic effects made mAbY.1 an unsuitable candidate for further development as a therapeutic agent. This example demonstrates that nonclinical safety studies may be essential for understanding off-target effects of mAbs prior to clinical trials.