Hypromellose phthalate HP-55 is an enteric coating polymer with a dissolution onset at pH 5.5. The grade is specified against the current USP/NF Hypromellose Phthalate monograph and the corresponding Ph. Eur. Hypromellose Phthalate monograph. In pharmaceutical processing, HP-55 is selected for gastric acid protection because the phthalyl ester substituents remain largely unionized below pH 5.5, limiting water uptake and drug release, while ionization in intestinal buffer leads to swelling and dissolution. The material is commonly handled as a white to off-white granular powder and requires organic or mixed organic-aqueous solvent systems for film formation. Parenteral administration is not an established application for this grade; no compendial monograph supports its use in injectable formulations, and the scenarios below are confined to oral solid dosage forms.

Why HP-55 Coatings Fail Below pH 5.5 and What That Imposes on Tablet Core Design

Tablet coating with HP-55 begins with preparation of a 7–10% w/w polymer solution in acetone/water or acetone/ethanol/water. The solution is held at 20–25°C and passed through a 100 µm screen to remove undispersed polymer aggregates and foreign particulates that would otherwise block two-fluid nozzles. Plasticizer selection is more influential than polymer concentration because HPMCP films have a relatively high glass transition temperature and can crack under mechanical stress if unplasticized. Triethyl citrate is added at 10–20% of polymer solids; acetyl triethyl citrate may substitute when plasticizer migration into the tablet core is a stability concern. Plasticizer loads above 25% of polymer solids produce tacky films and can extend buffer-stage dissolution lag time, while loads below 10% may generate brittle films that fail after compression or during handling. Talc at 10–30% of polymer solids and titanium dioxide at 5–10% of polymer solids are typical anti-tack and opacification solids. Coating is performed in side-vented coating pans fitted with two-fluid nozzles. Representative starting parameters include inlet air at 40–60°C, exhaust air at 30–40°C, product bed temperature at 28–34°C, atomization pressure at 0.8–1.5 bar, and spray rate at 5–15 g/min/kg tablet bed. Tablet cores should have friability below 0.8% and should be dedusted before loading to reduce nozzle blockage and edge film irregularity. Core formulations containing highly alkaline excipients such as magnesium oxide or high-buffering carbonate systems present a process conflict because interfacial pH can rise above 5.5 and trigger premature polymer ionization, leading to pitting or reduced gastric resistance. Coating weight gain of 8–12% is standard for convex cores; deep concave or embossed cores require 12–15% weight gain and reduced pan speed to avoid film erosion at edges. After spraying, curing at 40°C for 1–2 h reduces residual moisture and film tack. Acid resistance is verified by USP <711> with 0.1 N HCl at pH 1.2 for 2 h, during which release should not exceed 10% of label claim, followed by buffer-stage testing at pH 6.8. Disintegration testing according to USP <701> and residual solvent testing according to USP <467> and ICH Q3C complete the release package. Terminal product types include enteric-coated tablets, enteric-coated mini-tablets, and bilayer tablets in which the HP-55 film serves as the functional barrier for acid-labile active ingredients.

Pellet coating for multiparticulate capsules is carried out in a bottom-spray fluid bed fitted with a Wurster column. The substrate is microcrystalline cellulose spheres or sugar spheres with particle size 710–850 µm for capsule filling and 500–710 µm for sachets. A coating solution of HP-55 at 6–9% w/w solids in acetone/water is sprayed through a two-fluid nozzle at 8–20 g/min/kg, with inlet air at 50–65°C, product temperature at 32–38°C, atomization pressure at 1.2–2.0 bar, and chamber relative humidity below 30%. Inadequate control of chamber humidity can cause polymer precipitation in the nozzle and uneven film deposition on the lower Wurster zone. Plasticizer loading is 15–20% of polymer solids, and talc is 10–20% of polymer solids. Total polymer weight gain is 12–18% relative to uncoated pellets; the higher end is required when coated pellets will be compressed into tablets or filled into capsules with limited moisture barrier. Curing at 40°C for 2 h reduces film tack and supports uniform gastric resistance. Multiparticulate systems preserve enteric protection even if a capsule shell fractures or a tablet disintegrates, which is a critical performance distinction from monolithic tablet coatings. Compliance is anchored to USP <711> for delayed-release capsules, USP <905> for content uniformity, and product-specific dissolution in pH 5.8–6.8 buffers. Terminal products include enteric-coated pellet-filled hard gelatin capsules, enteric-coated pellet-filled HPMC capsules, and multi-unit pellet systems for modified release.

Enteric Granule Preparation for Sachet and Dry Syrup Products

Granules intended for reconstitution or direct dosing have higher specific surface area and lower mechanical strength than spherical pellets, so the coating operation must tolerate granule attrition and dust generation. Granules are prepared by wet granulation or fluid-bed granulation to a sieve cut of 500–1000 µm, then coated in a top-spray fluid bed or rotor granulator. The coating solution is prepared at 5–8% w/w polymer solids because lower viscosity improves fine droplet atomization on irregular granule surfaces. Triethyl citrate is added at 10–20% of polymer solids; talc is increased to 20–30% of polymer solids to suppress granule-to-granule sticking. Inlet air is limited to 45–55°C, product temperature to 30–35°C, atomization pressure to 1.0–1.8 bar, and spray rate to 5–12 g/min/kg. Higher spray rates cause agglomeration because the high surface area of granules promotes rapid solvent uptake and film surface wetting. Coating weight gain of 10–15% is standard; above 18% weight gain, agglomeration risk rises sharply and the fraction of fine particles retained in the filter may increase. After coating, granules are passed through a 1000 µm screen to remove oversize agglomerates. Acid-stage release is tested by USP <711> with an acid phase of 0.1 N HCl for 2 h; buffer-stage release may use pH 5.5 or 6.0 to match the intended gastrointestinal site. Additional compliance measures include Ph. Eur. 2.9.3 dissolution testing and USP <731> loss on drying. Terminal products are enteric-coated granules in sachets, dry syrup powders for reconstitution, and sprinkle granules intended for paediatric or geriatric administration.

Spray-dried amorphous solid dispersions for poorly water-soluble oral active ingredients use HP-55 as a pH-dependent crystallization inhibitor. Active:polymer weight ratios are screened from 1:1 to 1:3, with total spray solution solids at 2–5% w/w in acetone or acetone/water to maintain viscosity below 10 cP. The solution is filtered through 0.45 µm or finer filtration before spray drying to remove undissolved particles that would otherwise act as crystal nuclei. Closed-loop spray dryers with inert-gas mode and explosion-proof construction are required for acetone-containing feeds. Inlet temperature is set at 80–110°C, exhaust temperature at 50–65°C, and condenser temperature at −10 to −20°C for solvent recovery. The resulting dispersion is vacuum-dried at 40°C for 12–24 h to reduce residual acetone below ICH Q3C limits. HP-55 suppresses active ingredient crystallization during solvent evaporation and provides pH-dependent release: low release in 0.1 N HCl and rapid release in pH 5.5–6.8 buffers. Solid-state stability is verified by powder X-ray diffraction and modulated differential scanning calorimetry; performance is tested by USP <711> and USP <467> for residual solvents. Terminal products include tablets, capsules, and film-coated tablets manufactured from the spray-dried intermediate. Published data for specific active-polymer systems is limited, and the drug-polymer ratio must be established through phase-solubility, supersaturation, and stability studies rather than assumed from the polymer alone.

When Delayed-Release Multiparticulates Are Compressed into Orodispersible Tablets

Tableting enteric-coated pellets requires a cushioning excipient matrix to preserve film integrity under compression. Coated pellets with an HP-55 weight gain of 12–18% are blended with mannitol, microcrystalline cellulose, crospovidone, and sodium stearyl fumarate. The coated pellet fraction is typically 40–60% w/w of the final tablet mass; higher fractions increase acid-stage release because pellet-to-pellet contact creates local stress concentrations that fracture the enteric film. Compression is performed on a rotary tablet press with precompression at 5–10 kN and main compression at 10–15 kN. Tablet breaking force is maintained above 30 N to avoid cap cracking, but not above 70 N to limit polymer film fracture. Compression speed is reduced to 20–40 rpm on a 10-station press, and feeder paddle speed is lowered to minimize pellet segregation. After compression, tablets are tested for disintegration in simulated saliva pH 6.8 or water at 37°C; oral disintegration time below 30 s is common for correctly formulated orodispersible tablets. Acid-stage release is measured by USP <711> and should remain below 10% in 0.1 N HCl after 2 h. Tablet hardness tests follow USP <1062>, and disintegration follows USP <701>. Plasticizer selection in the pellet coating is critical because film fracture under compression cannot be corrected by downstream processing. The terminal dosage type is an orodispersible tablet containing enteric-coated microparticles, suitable for paediatric and geriatric patients with swallowing difficulty.