1-(3-(((2,5-Dioxopyrrolidinyl)Oxy)Carbonyl)Phenyl)-1H-Pyrrole-2,5-Dione

Maleimide-NHS Carbonate as a Heterobifunctional Crosslinker in Antibody-Drug Conjugates: Process Windows and Conjugation Stoichiometry

Controlled bioconjugation of cytotoxic payloads to monoclonal antibodies (mAbs) using 1-(3-(((2,5-Dioxopyrrolidinyl)Oxy)Carbonyl)Phenyl)-1H-Pyrrole-2,5-Dione requires strict management of pH and temperature gradients to prevent maleimide ring hydrolysis and premature NHS carbonate aminolysis. The molecule’s phenyl spacer imparts moderate hydrophobicity, influencing aggregation propensity during aqueous conjugation—a parameter routinely monitored via dynamic light scattering (DLS) on a Malvern Zetasizer Nano ZS at 25 °C with backscatter detection at 173°. In a typical manufacturing protocol, the mAb ( 5–10 mg/mL in phosphate-buffered saline, pH 7.2 ± 0.1 ) is first reduced with tris(2-carboxyethyl)phosphine (TCEP) at 2.5–3.0 molar equivalents per interchain disulfide, quenched, and the maleimide-NHS carbonate linker added at a 2.2–2.8 molar excess relative to free thiols. NHS carbonate reactivity toward lysine ε-amines is pH-dependent: the conjugation must proceed at pH 7.0–7.4 to balance maleimide selectivity and NHS ester half-life, which decreases from approximately 30 minutes at pH 7.0 to under 5 minutes at pH 8.5 in 100 mM phosphate at 20 °C, as quantified by reverse-phase HPLC monitoring of NHS release at 260 nm.

The sequential, one-pot approach—thiol-maleimide coupling followed by direct addition of amine-containing payload—is complicated by the carbonate linkage's susceptibility to intramolecular cyclization if residual moisture or hydroxyl species are present. Facilities operating under ICH Q7 for active pharmaceutical ingredient (API) manufacture pre-dry the linker in vacuum ovens ( ≤ –0.08 MPa , 30 °C ) for a minimum of 12 hours and handle it in gloveboxes with dew points below –40 °C . Drug-to-antibody ratio (DAR) is characterized using hydrophobic interaction chromatography (HIC) on a TSKgel Butyl-NPR column with a 1.5 M to 0 M ammonium sulfate gradient over 20 minutes . Typical DAR values obtained range from 3.8 to 4.2 for interchain cysteine conjugates, while site-specific engineered cysteine constructs achieve DAR 1.9–2.0 with homogeneity exceeding 95% . The carbonate-derived carbamate linkage between linker and payload exhibits pH-dependent stability: in human plasma at 37 °C , < 10% payload release was observed over 7 days at pH 7.4 , whereas lysosomal pH 5.0 accelerates hydrolysis with a t₁/₂ of approximately 18 hours (reported in Bioconjugate Chemistry stability studies with surrogate substrates). This pH-labile profile is exploited for intracellular drug release while minimizing systemic toxicity.

Regulatory dossiers for ADC intermediates referencing this linker require compliance with the ICH M7(R2) guideline for mutagenic impurities, as the maleimide moiety is a structural alert in silico (DEREK Nexus 6.3.0 ) and must be controlled below the threshold of toxicological concern (TTC) of 1.5 µg/day . Process validation batches routinely include LC-MS/MS limit tests for free maleimide-NHS carbonate at ≤ 5 ppm relative to the ADC. Equipment cleaning validation adheres to 21 CFR 211.67 and employs swab recovery studies with detection limits below 0.1 µg/cm² .

Production of site-specific ADCs using transglutaminase-mediated enzymatic conjugation with this linker has been demonstrated on ÄKTA pilot-scale chromatography systems with inline UV/Vis and conductivity monitoring. The reaction requires 0.5 U/mL microbial transglutaminase (mTG) in 50 mM Tris-HCl, pH 8.0, supplemented with 10% v/v dimethylformamide to maintain linker solubility, achieving coupling efficiencies above 85% within 4 hours at 25 °C . Residual mTG is removed via Protein A affinity capture with a 5 mL HiTrap MabSelect SuRe column at a linear flow rate of 150 cm/h .

When the Formula Calls for a Cleavable Conjugator: Linking Amine-Containing Polymers to Thiol-Modified Nanoparticles

Poly(lactic-co-glycolic acid) (PLGA) nanoparticles surface-functionalized with cysteine-terminated poly(ethylene glycol) (PEG-SH, M_w 2000–5000 Da) are commonly activated with 1-(3-(((2,5-Dioxopyrrolidinyl)Oxy)Carbonyl)Phenyl)-1H-Pyrrole-2,5-Dione to introduce amine-reactive handles for targeting ligands or fluorescent probes. The reaction is executed in an aqueous/organic biphasic system: freshly prepared nanoparticles suspended in 0.1 M sodium borate buffer, pH 8.2 , are treated with a 0.5–2.0 molar excess of the heterobifunctional crosslinker dissolved in anhydrous dimethyl sulfoxide (DMSO) at a final organic phase fraction not exceeding 5% v/v to avoid nanoparticle aggregation. Mixing is performed on a vortexer at 2500 rpm for 15 seconds every 2 minutes over a total reaction time of 30 minutes , with temperature maintained at 4 °C through an ice-water bath to slow maleimide ring-opening. The resulting maleimide-activated intermediate is isolated by centrifugal filtration (Amicon Ultra-100 kDa MWCO, 4000 × g, 10 minutes ) and immediately incubated with the amine-functionalized ligand (e.g., transferrin, RGD peptide, Cy5-amine) at a molar ratio of 5:1 ligand-to-nanoparticle in PBS pH 7.4 for 2 hours at 25 °C .

The stability of the carbamate linkage under physiological conditions becomes a critical design parameter. Accelerated stability studies performed per ICH Q1A(R2) at 40 °C / 75% RH over 6 months show that the PEG shell remains > 95% intact when analyzed by ¹H NMR in D₂O integrating the PEG methylene signal at δ 3.65 ppm . However, exposure to 0.1 M NaOH for 4 hours at 37 °C results in complete cleavage of the linker at the carbonate junction, confirming base-labile degradation that can be leveraged for triggered release studies. This reactivity profile distinguishes it from non-cleavable SMCC (4-(N-maleimidomethyl)cyclohexane-1-carboxylic acid N-hydroxysuccinimide ester)-derived conjugates, where the cyclohexane spacer is stable under the same alkaline conditions. Published data for this specific configuration is limited regarding in vivo biodegradation kinetics in tumor microenvironments; however, benchtop enzymatic challenges with porcine liver esterase (PLE, 50 U/mL in HEPES buffer, pH 7.4 ) indicate > 60% hydrolysis within 24 hours .

Quality control for nanoparticle conjugates includes endotoxin testing per USP <85> using Limulus amebocyte lysate (LAL) with a sensitivity of 0.06 EU/mL , and free maleimide-NHS carbonate quantification via HPLC-UV at 254 nm with a C18 column and a 0.1% trifluoroacetic acid/acetonitrile gradient. The acceptance level for unreacted crosslinker is set at < 0.1% of total linker fed . Particle size stability is monitored by nanoparticle tracking analysis (NTA) on a NanoSight NS300 instrument; z-average diameter shifts exceeding 20 nm trigger batch rejection.

In a parallel application, amine-terminated polycaprolactone (PCL-NH₂) chains are dissolved in anhydrous tetrahydrofuran (THF) at 10% w/v and reacted with the linker at a 1.05:1 molar ratio (NHS carbonate:amine) in the presence of 1.1 equivalents of triethylamine as acid scavenger. After 24 hours stirring under argon, the maleimide-terminated PCL is precipitated in cold diethyl ether and dried under vacuum. Subsequent thiol-ene coupling with a thiol-modified poly(2-methyl-2-oxazoline) yields a block copolymer self-assembling into micelles with a critical micelle concentration (CMC) of 8.2 × 10⁻⁷ M determined by pyrene fluorescence (ex 334 nm, em 373/384 nm ).

Crosslinking Density and Swelling Ratios in Injectable Hydrogels for Cartilage Tissue Regeneration

Maleimide-NHS carbonate functions as a tetherable crosslinker in multi-arm PEG hydrogels where thiolated hyaluronic acid (HA-SH, degree of substitution 20–40% ) and amine-terminated gelatin (denatured collagen, Bloom strength 150–250 ) are co-crosslinked in situ. The crosslinking process is initiated by mixing two precursor solutions in a dual-barrel syringe fitted with a static mixer (Mixpac 2.5 mL , 1:1 ratio ): barrel A contains HA-SH in PBS, pH 6.8 , while barrel B contains gelatin-amine pre-functionalized with the heterobifunctional linker at a carbonate:amine stoichiometry of 1.2:1 . The maleimide-to-thiol ratio in the final mixture is maintained at 1.0 ± 0.05 to minimize unreacted maleimide groups that could react with cysteine residues of surrounding tissue proteins in vivo. Gelation time measured by oscillatory rheometry (TA Instruments AR-G2, 40 mm parallel plate, 0.5 mm gap, 1 Hz, 1% strain ) at 37 °C is consistently between 45 and 90 seconds , allowing sufficient working time for injection through a 21G needle before the storage modulus (G') crosses over loss modulus (G").

Equilibrium swelling ratios in PBS at 37 °C after 24 hours range from 28 to 42 depending on HA-SH content ( 1–4% w/v ). Compressive modulus, tested on an Instron 3345 universal testing machine with a 10 N load cell at a crosshead speed of 1 mm/min , increases from 12 kPa to 68 kPa as crosslink density rises—well within the range for human articular cartilage ( 30–80 kPa at 10–20% strain ). The carbamate bonds introduced between the linker and gelatin amines demonstrate gradual enzymatic degradability: incubation in collagenase type II ( 0.5 U/mL in Tris-buffered saline with 5 mM CaCl₂ , pH 7.4 ) degrades 50% of the hydrogel mass within 10 days , versus 22 days for SMCC-derived amide-linked controls. This rate differential positions the carbonate linkage as a mid-degradation-rate element in hydrogel bioresorption profiles.

First-in-human implant considerations demand biocompatibility evaluation per ISO 10993-5:2009 (cytotoxicity, MTT assay using L929 fibroblasts, extract dilution 1:1 to 1:8 ) and ISO 10993-10:2021 (skin sensitization, Guinea pig maximization test). Leachable HPLC analysis must confirm free linker concentration below 0.05 µg/mL in 24-hour PBS extracts. Sterilization compatibility was verified by exposing lyophilized linker powder to gamma irradiation at 25 kGy ; ¹H NMR purity decreased from 99.2% to 97.8% with no new genotoxic impurities above 0.1% as per LC-MS profiling.

Can This Bifunctional Linker Withstand Continuous Resin-Bound Peptide Synthesis Conditions?

Solid-phase peptide synthesis (SPPS) incorporating 1-(3-(((2,5-Dioxopyrrolidinyl)Oxy)Carbonyl)Phenyl)-1H-Pyrrole-2,5-Dione as a side-chain modifier or N-terminal cap for subsequent chemoselective ligation requires careful selection of a resin and cleavage cocktail that do not compromise the carbonate junction. Fmoc-based SPPS on a Liberty Blue 2.0 microwave peptide synthesizer (CEM Corporation) typically exposes the growing peptide to 20% piperidine in dimethylformamide (DMF) for deprotection cycles lasting 3–5 minutes at 90 °C . Under these conditions, the NHS carbonate moiety undergoes rapid aminolysis by piperidine, releasing the free alcohol (3-hydroxyphenyl maleimide) within seconds. Therefore, the linker is not installed on the peptide while still attached to the resin, unless a fully protected, acid-labile resin (e.g., 2-chlorotrityl chloride) is employed and piperidine deprotection is substituted with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) 2% in DMF for 2 × 2 minutes at 25 °C —a strategy that reduces premature carbonate cleavage.

The preferred workflow involves post-synthesis solution-phase conjugation. The purified peptide, possessing a single free cysteine (C-terminal or via incorporation of an orthogonally deprotected Cys(StBu) residue), is dissolved in degassed 100 mM sodium acetate buffer, pH 5.5, containing 2 mM EDTA and 5% w/v trehalose as a lyoprotectant. The maleimide-NHS carbonate linker is added at 1.5 equivalents relative to free thiol from a 100 mM stock in anhydrous DMSO. After 2 hours at 22 °C with gentle rotation, maleimide conjugation is quantified by DTNB assay; typical yields exceed 92% . The resulting peptide-linker intermediate is desalted on a Sephadex G-25 column (HiTrap Desalting, 5 mL ) into 0.1 M NaHCO₃, pH 8.3, and immediately combined with the amine-containing partner (e.g., amino-functionalized oligonucleotide, PEG-amine, or fluorophore) at a 2:1 molar ratio. NHS ester-reactive impurities present in the buffer (Tris, ammonium ions) must be absent; otherwise, quenching side reactions reduce coupling efficiency to below 40% .

Quality assessment of the conjugate includes high-resolution ESI-MS (deconvolution accuracy ± 1 Da) and analytical size-exclusion chromatography (SEC) on a Superdex 75 Increase 10/300 GL column at a flow rate of 0.5 mL/min to confirm absence of high-molecular-weight aggregates. Stability of the carbamate linkage following lyophilization and reconstitution is tested per ICH Q6B : after storage at 2–8 °C for 12 months , peak area purity remains > 96% . The maleimide-thioether adduct completes its characteristic retro-Diels-Alder degradation in solution at pH > 8.5 beyond 48 hours ; hence, all formulations must be maintained at pH ≤ 7.4 for extended storage.

Evaluating Surface Plasmon Resonance Chip Functionalization Using an Amine-Thiol Coupling Strategy

On Biacore T200 sensor chips (CM5, Series S), amine coupling via EDC/NHS chemistry is standard, yet the incorporation of a thiol-reactive maleimide allows oriented immobilization of cysteine-tagged proteins while the carbonate linkage provides an amine-reactive anchor for the carboxymethyl dextran surface. A two-step protocol has been validated in production environments: after standard EDC/NHS activation ( 7-minute injection of 0.4 M EDC / 0.1 M NHS at 10 µL/min ), the linker is injected at a concentration of 0.5 mM in 10 mM sodium acetate, pH 4.5 for 420 seconds (contact time). This pH ensures the maleimide remains stable while the NHS carbonate reacts with the amino groups introduced onto the dextran via ethylenediamine grafting (pre-immobilization step). The resulting maleimide-functionalized surface is then used immediately for cysteine-tagged ligand capture. The carbamate bridge stabilizes the attachment under continuous-flow conditions (30 µL/min , 25 °C ) for over 200 regeneration cycles using 10 mM glycine-HCl, pH 2.0 , with less than 2% signal drift as measured by baseline R_max variation.

Affinity measurements (K_D) generated with this surface deviate by no more than 1.5-fold from free-solution binding kinetics as verified by kinetic exclusion assay (KinExA 3200). Validation per USP <1032> (Design and Development of Biological Assays) requires that the immobilization level remain within 3000–5000 RU for analytes of 15–50 kDa to prevent mass transport limitation; this is achieved by adjusting linker concentration and contact time within the ranges above. Residual NHS ester after immobilization is quenched with a 1 M ethanolamine-HCl, pH 8.5, 7-minute injection, followed by 50 mM cysteine in 0.1 M acetate, pH 5.0 to block unreacted maleimide sites.

Sensor chip preparation is performed on a Biacore S200 using the automated Immobilization Wizard, and the reagents must be filtered through 0.22 µm syringe filters prior to loading. A troubleshooting note: in laboratories where relative humidity in the instrument bay exceeds 60% , NHS carbonate hydrolysis accelerates, evidenced by a decline in immobilization capacity over a single day (up to 30% reduction after 8 hours ); pre-dehumidification of the instrument environment to RH ≤ 45% or aliquoting fresh linker solution every 4 hours resolves this drift.

What Limits the Use of this Linker in Photopatternable PEG-Diacrylate Hydrogels for Microfluidic Cell Culture?

Pre-functionalization of a PEG-diacrylate (PEGDA, M_n 700 Da ) prepolymer solution with this heterobifunctional crosslinker introduces amine-reactive handles without interfering with radical photopolymerization. The linker is dissolved in the prepolymer mix at 1.0–2.5 wt% relative to PEGDA along with the photoinitiator lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) at 0.05 wt% . Photomask lithography using a collimated LED UV source ( 365 nm , 20 mW/cm² , exposure 90 seconds ) yields microchannel structures with vertical sidewalls and 200 µm feature resolution. Importantly, the NHS carbonate does not generate reactive radicals under UV exposure; LC-MS analysis of the photo-cured hydrogel after solvent extraction shows > 97% maleimide intactness. After photopatterning, the channel surfaces are accessible for functionalization: a solution of fibronectin ( 50 µg/mL in PBS, pH 7.4 ) is flowed through the microchannels at 2 µL/min for 2 hours , depositing a uniform protein coating verified by fluorescent imaging (Alexa Fluor 488-labeled fibronectin) with a coefficient of variation in intensity of < 15% across a 1 cm channel length.

The limiting factor in this application is the gradual solvolysis of the carbamate linkage in aqueous cell culture medium (DMEM with 10% FBS , 37 °C , 5% CO₂ ). Over 14 days , approximately 25% of the immobilized fibronectin is released into supernatant as quantified by ELISA, correlating with a drop in cell adhesion force (single-cell micropipette aspiration) from 45 nN to 32 nN . For long-term (> 3 week ) co-culture experiments, the hydrolytic instability restricts the scaffold’s functional lifetime; however, for acute chemotaxis assays (3–7 days) the degradation is negligible. The device fabrication workflow is performed in a ISO Class 7 cleanroom, and the photomasks are chrome-on-quartz with feature sizes down to 5 µm .