# Chapter 6 — Immobilized Biocatalysis Delivers a Credible Path from Lab Prototype to GMP Candidate for GalNAc Conjugation

Three parallel developments, converging between 2020 and 2026, establish immobilized biocatalysis as the most technically credible route to replacing chemical protecting-group strategies in GalNAc conjugation for dual-target siRNA: the SUGAR-TARGET glycosyl-transferase cascade (Makrydaki et al., *Nat Chem Biol* 2024) demonstrating four-cycle enzyme reuse over 80+ hours with >70% retained activity [src_C05]; the CLEA-LentiKats lipase formulation accumulating 10 g product per liter over at least six continuous-flow cycles in deep eutectic solvents (DES) [src_C10]; and Codexis ECO's immobilized polymerase/phosphatase reactor achieving >98% coupling efficiency with oligonucleotides at 6 mM substrate concentration [src_B11]. These routes now occupy TRL 5–7, up from TRL 3–4 before 2022 — close enough to GMP readiness (TRL 8–9) that the remaining gap is regulatory process-validation documentation, not fundamental chemistry.

The strategic case for dual-target siRNA is direct. Each additional GalNAc arm — from triantennary (3×) to tetraantennary (4×) and beyond — multiplies protecting-group manipulation steps in chemical synthesis. An immobilized glycosyl-transferase that installs the terminal GalNAc residue with >95% conversion sidesteps both the atom-economy penalty and the ICH Q3D copper-residue burden that makes CuAAC click chemistry difficult to justify at commercial scale [src_C08, src_C09].

## 6.1 SUGAR-TARGET Glycosyl-Transferase Cascade: Four-Cycle Reuse Validates the Architecture

The SUGAR-TARGET platform arranges four immobilized enzymes — GnTI, ManII, GalT, and SiaT — in sequential spatiotemporal compartments on streptavidin-coated silica beads [src_C05]. The biotin–streptavidin immobilization method exploits in vivo biotinylation (BirA/AviTag), enabling one-step immobilization and purification directly from E. coli lysate, with >65% biotinylation yield for GnTI and GalT and >85% for SiaT [src_C05]. There is no detectable enzyme leaching from the beads — a critical quality attribute for APIs that must meet HCP and ICH Q3D residual limits [src_C05].

Operational stability data from GalT reusability experiments are the key performance anchor. Immobilized GalT retained over 70% of its initial activity after four cycles spanning more than 80 hours of cumulative operation, with terminal galactosylation of CHO-derived h-IgG reaching 97.4% after the first cycle and remaining at 84% after the fourth [src_C05]. Each step in the cascade achieved >95% conversion to the desired glycoform. Activity decrease was attributed to small enzyme loss during wash steps, not denaturation.

For translation to GalNAc-siRNA manufacturing, the substrate shifts from a glycoprotein IgG to a short oligonucleotide (21-mer, ~6–8 kDa). Reduced steric occlusion of the enzyme active site by an oligonucleotide versus a full IgG Fc domain suggests conversion rates could exceed the 95% demonstrated with macromolecular substrates [src_C05, src_C09]. The cofactor requirement (UDP-GalNAc, UDP-Gal) is addressed via established nucleotide-sugar regeneration cascades that can be co-run in parallel loops [src_C09]. The 2025 extension using SpyCatcher/SpyTag-immobilized Leloir glycosyltransferases on maleimide-activated agarose showed immobilization yields of 67–100% across five GT variants, reusability for six reactions over three consecutive days, and specific activities ranging from 285 mU·mg⁻¹ (SpyC-β4GalT) to 4,734 mU·mg⁻¹ (SpyC-GTA/R176G), with several variants actually gaining activity at one month (SpyC-β4GalT: 138% of Day 1) due to conformational stabilization on-support [src_G01].

Support material selection matters for scale-up. SUGAR-TARGET used silica beads for free-glycan reactions (mechanically rigid, moderate-backpressure compatible) and magnetic particles for protein substrates (rapid magnetic decantation replaces centrifugation) [src_C05]. For packed-bed reactor configuration, methacrylate copolymer beads — rigid, available with 20–80 mg protein loading per gram dry support, 60–85% activity retention post-covalent attachment — are the preferred alternative to agarose, which compresses under backpressure [src_C08].

## 6.2 CLEA Lipase in DES: Single-Step Desymmetrization Eliminates Protecting-Group Chemistry

Chemical synthesis of 2-acetamido-2-deoxy-D-galactose (GalNAc) derivatives for siRNA conjugation requires three to five protecting-group steps per arm, compounding to ≤41% overall yield across a 4–6-step sequence [src_C10]. CLEA lipase desymmetrization in DES condenses this to one or two enzyme steps, with ee values for N-acetylhexosamine diacetate substrates reported at 93–>99% depending on DES composition and substrate concentration [src_C09]. Atom economy improves 40–60% versus the chemical route by eliminating Ac₂O, TfOH, and deprotection base stoichiometry [src_C10].

The CLEA-LentiKats format (Guajardo et al., *J Biotechnol* 2020) immobilizes Candida antarctica lipase B first as a CLEA via glutaraldehyde crosslinking, then entraps the aggregate in LentiKats polyvinyl alcohol (PVA) hydrogel particles [src_C10]. Adding 20% (v/v) aqueous buffer as co-solvent lowers DES viscosity enough for pump-driven continuous flow while maintaining enzyme stability. The format demonstrated ≥6 operational cycles accumulating 10 g product per liter under non-optimized conditions — 3–4× higher space-time yield than equivalent solution-phase reaction due to the higher substrate concentration achievable in DES (operating window: 50 mM to 1 M substrate, compared to 0.1–10 mM for cofactor-dependent GTs) [src_C10].

Flow-reactor suitability for CLEA-LK lipase is high. Residence-time distribution in a packed bed of LentiKats lenticular beads (~1–2 mm) approximates plug flow, enabling residence-time control to the point of maximum ee — avoiding the over-reaction racemization that degrades ee in stirred-batch reactors. Support compatibility is limited to DES-insoluble, mechanically robust materials: LentiKats (cross-linked PVA) and epoxy-methacrylate copolymer qualify; standard silica and agarose do not [src_C08, src_C10]. The regulatory challenge for DES processes is solvent characterization: choline chloride/urea (reline) and choline chloride/glycerol are not classified by ICH Q3C, requiring a custom acceptable daily intake calculation for any IND package.

## 6.3 Flow and Microgel Formats Add Productivity but Introduce PAT Complexity

The ACS Biomacromolecules 2024 paper (src_C13) demonstrates droplet-microfluidics-produced polymer microgels (~100 µm diameter) encapsulating SpyCatcher-linked β4GalT and β3GlcNAcT [src_C13]. SpyCatcher/SpyTag covalent conjugation ensures irreversible enzyme binding, eliminating leaching. A tandem cascade of β4GalT and α3GalT inside microgels produced target glycan at high yield, paving the way for a modular membrane bioreactor for continuous glycan synthesis [src_C13].

Productivity advantage is estimated at 10–50× over batch at equivalent enzyme loading, based on the elimination of batch setup, wash, and centrifugation time — typical batch glycosyl-transfer cycles run 2–16 hours per reaction; continuous-flow microgel reactors reach steady-state within two reactor volumes then operate uninterrupted [src_C13, src_C09]. The regulatory barrier from TRL 6 to GMP is process analytical technology (PAT) per ICH Q13: inline conversion monitoring, residual enzyme surveillance, and particle-integrity monitoring must each be validated — a 12–18-month development timeline per product at GMP scale [src_C08].

## 6.4 TRL Map: ECO Synthesis Leads, Glycosyl-Transfer Cascades Need 24 More Months

The current TRL landscape assigns distinct positions to each route:

| Biocatalytic Step | Immobilization Method | Reuse Data | Support Material | Space-Time Yield | TRL (2026) |
|---|---|---|---|---|---|
| GT cascade (SUGAR-TARGET-type) | Biotin–streptavidin / silica or magnetic | 4 cycles, >80 h | Silica / magnetic particles | Not quantified at scale | TRL 6–7 |
| Lipase desymmetrization (CLEA-LK) | CLEA + PVA entrapment | ≥6 cycles | LentiKats PVA / methacrylate | 10 g product/L | TRL 5–6 |
| Flow-format GT (microgel) | SpyCatcher covalent | 6 reactions / 3 days | Polymer microgel | 10–50× vs. batch (est.) | TRL 5–6 |
| ECO sequential synthesis + conjugation | Enzyme on resin, oligo in solution | Not disclosed | Proprietary resin | Targets >10 kg/run | TRL 7 |

Codexis ECO leads on TRL. The March 2026 agreement to manufacture 50 g siRNA for a cardiovascular preclinical program confirms first commercial manufacturing engagement [src_E43]. The platform operates at 6 mM oligonucleotide with enzymes immobilized on proprietary resin, achieves >98% coupling efficiency, and scaled ligation workflows tolerate up to 100 g/L substrate with engineered ligases achieving >95% conversion [src_B11]. Platform-level claim of >10 kg per run with technology transfer to GMP sites positions ECO at TRL 7 transitioning to TRL 8 [src_B11].

The gaps between TRL 7 and TRL 9 (GMP commercial readiness) are well-defined. For immobilized glycosyl-transferase cascades: (1) enzyme residual specification development — no pharmacopeial limit for biocatalyst HCP in oligonucleotide APIs currently exists; method development per ICH Q2(R1) is required; (2) UDP-sugar cofactor residue control — target <1 ppm by LC-MS/MS, achievable by anion-exchange polishing [src_C09]; (3) support leachable characterization — glutaraldehyde from CLEA preparation requires ICH Q3C Class 3-equivalent control; (4) lot-to-lot enzyme consistency — commercially available GTs currently show 15–40% inter-lot specific activity variation, requiring upstream manufacturing standardization [src_G01]. For CLEA lipase: DES-solvent classification and GalNAc-specific substrate validation add ~12 months to the TRL 8 timeline.

Codexis's trajectory from TRL 5 (~92% average incorporation efficiency at TIDES EU 2023) to TRL 7 (first commercial manufacturing agreement, March 2026) took approximately 28 months [src_B11, src_E43]. A well-resourced entrant with validated enzyme lots and a drug-substance partner can replicate TRL 6 → TRL 8 in 24 months — the constraint is regulatory documentation, not catalytic performance.

## Counter-Evidence

**Scale-up fundamentals for SUGAR-TARGET remain unvalidated.** All four-cycle reusability data derive from mg-scale, sub-2 mL reaction volumes [src_C05]. Packed-bed column scale-up at 100 mL–1 L will introduce bead attrition, channeling, and pressure-drop effects invisible at lab scale. Silica bead fines generated under mechanical stress contaminate product and degrade enzyme loading per gram over successive regenerations [src_C08]. TRL 7 within two years for GT cascades is plausible but conditional on lab-to-column scale-up data that do not yet exist.

**UDP-sugar cofactor cost challenges economic viability at scale.** UDP-GalNAc research-grade pricing is $200–500/g, compared to <$1/g for GalNAc itself [src_C09]. For a tetraantennary dual-target siRNA construct (4 GalNAc per strand × 2 strands), cofactor demand at 100 g/batch scale is substantial. If enzymatic regeneration efficiency falls below 80%, the cost advantage over chemical synthesis disappears — a limitation acknowledged explicitly in the SUGAR-TARGET paper [src_C05].

**No regulatory precedent for immobilized-enzyme GalNAc conjugation in approved siRNA.** All seven FDA-approved GalNAc-siRNA drugs (as of March 2025) used chemical phosphoramidite synthesis with chemical conjugation [src_E01]. The first IND using immobilized-enzyme bioconjugation will face elevated scrutiny. NMPA 2026 chemoenzymatic guidance (src_B18) provides a drafting framework but is not yet final; the regulatory position on continuous-flow enzyme reactors for oligonucleotide bioconjugation specifically has not been tested [src_B18].

**ECO Synthesis targets full siRNA strand synthesis, not GalNAc cluster assembly.** The documented ECO advantage is sequential RNA extension; the GalNAc targeting moiety attachment chemistry in the March 2026 agreement is undisclosed [src_E43]. If the conjugation step uses chemical ligation, ECO's biocatalytic scope does not cover the full GalNAc-conjugation pipeline.
