Novel drug delivery systems

Pharmacy (B.Pharm) · Pharmaceutics · free study notes for exams, placements and interviews

In short

A novel drug delivery system (NDDS) is any formulation that controls the rate, time or place of drug release instead of simply dumping the dose like a conventional tablet or injection. The aim is to hold plasma concentration inside the therapeutic window for longer, cut dosing frequency and side effects, and carry drugs to a target site (tumour, lung, eye, colon). It is examined in B.Pharm Pharmaceutics (and the Industrial Pharmacy/NDDS paper), in GPAT and NIPER entrances, and in formulation-development interviews at Sun Pharma, Cipla, Dr. Reddy's, Lupin, Zydus, Torrent, Glenmark and Intas. India's generic and 505(b)(2) business runs heavily on modified-release products, so release-mechanism maths, polymer choice and dissolution testing are standard interview ground. Regulation sits with CDSCO under the Drugs and Cosmetics Act, 1940 with the New Drugs and Clinical Trials Rules, 2019, and manufacture under revised Schedule M (GMP).

Explained

Definitions, rationale and classification

Conventional (immediate release): whole dose available at once → sharp peak (risk of toxicity), fast trough (loss of effect), frequent dosing.
Sustained release (SR): slows release to prolong action, but the release rate itself is not strictly constant.
Controlled release (CR): releases at a predetermined, reproducible rate, ideally zero-order, often for a defined duration.
Delayed/targeted release: release postponed (enteric, colon-targeted) or directed to a site (ligand-tagged nanocarriers).
Prolonged / extended release (ER): the official (IP/USP) term for products giving at least a twofold reduction in dosing frequency.
Rationale: reduce Cmax–Cmin fluctuation, improve adherence, avoid first-pass metabolism (transdermal, buccal), protect labile drugs, localise action, reduce total dose and cost.
Classification by mechanism: (1) diffusion-controlled - matrix or reservoir; (2) dissolution-controlled - slowly dissolving coat or salt; (3) osmotically controlled; (4) ion-exchange resinates; (5) swelling/erosion-controlled hydrophilic matrices; (6) stimuli-responsive (pH, temperature, enzyme, magnetic); (7) vesicular and particulate carriers; (8) prodrug and chemical approaches.
Drug candidate criteria for CR: biological half-life roughly 2–8 h (t½ < 1 h needs an impractically large dose; t½ > 12 h is already long-acting), dose usually below ~500 mg, adequate aqueous solubility, absorption throughout the GI tract, not a narrow-therapeutic-index drug requiring tight titration, no dose-dependent (saturable) kinetics.
Poor candidates: drugs absorbed only in a narrow window, very large doses (e.g. several grams), drugs with active transport uptake limited to the duodenum.

Oral controlled-release systems

Matrix (monolithic) tablets: drug dispersed in a polymer. Hydrophilic matrices use HPMC (K4M, K15M, K100M), sodium CMC, xanthan, carbopol - on contact with water they hydrate to a gel layer, release is by diffusion plus erosion, giving square-root-of-time (Higuchi) to anomalous kinetics. Hydrophobic/insoluble matrices use ethyl cellulose, Eudragit RS/RL, waxes (carnauba, glyceryl behenate) - release by diffusion through pores or channels.
Reservoir (membrane-controlled) systems: drug core inside a rate-controlling polymer film or microcapsule; gives true zero-order release while the core stays saturated. Risk: dose dumping if the membrane cracks.
Osmotic pumps: semipermeable membrane (cellulose acetate) around an osmotically active core with a laser-drilled orifice. Water enters by osmosis, the saturated solution is pumped out at constant rate - zero order, independent of GI pH and motility. Elementary osmotic pump (soluble drug), push–pull OROS/bilayer (poorly soluble drug, with a polymeric osmotic push layer), controlled porosity osmotic pump (leachable pore formers instead of a drilled hole). Examples: nifedipine GITS, methylphenidate OROS.
Coated pellets / multiparticulates (MUPS): many small units of differing coat thickness in a capsule or tablet; failure of one pellet does not cause dose dumping; less affected by gastric emptying.
Ion-exchange resins: drug bound as a resinate; released by exchange with GI ions - rate depends on ionic strength, not pH alone.
Gastroretentive systems: floating (low-density, effervescent or swellable HPMC matrices), high-density sinking, mucoadhesive, swelling/plug-type, and magnetic. Useful for drugs absorbed in the upper gut or acting locally in the stomach (e.g. anti-*H. pylori*, ciprofloxacin, furosemide).
Colon-targeted systems: pH-dependent coats (Eudragit S100 dissolving above pH 7), time-dependent coats, microbially degraded polysaccharides (guar gum, pectin, chitosan, inulin) degraded by colonic bacteria, and pressure-controlled capsules. Used for ulcerative colitis (mesalazine, budesonide) and chronotherapy.
Pulsatile / chronotherapeutic: release after a programmed lag, matching circadian peaks (early-morning asthma and angina, rheumatoid stiffness) - rupturable-coat capsules, erodible plugs (Pulsincap-type), press-coated tablets.
Fast-dissolving / orodispersible forms are an improvement in convenience rather than rate control - they fall at the opposite end of the spectrum.

Transdermal, mucosal and parenteral depots

Transdermal drug delivery system (TDDS): drug permeates the stratum corneum (the rate-limiting barrier), reaching systemic circulation and bypassing hepatic first-pass metabolism.
Types: reservoir (drug solution/gel behind a rate-controlling membrane), drug-in-adhesive (simplest, thinnest), matrix/monolithic, and micro-reservoir hybrids. Layers: backing film, drug reservoir/matrix, rate-controlling membrane, pressure-sensitive adhesive, release liner.
Ideal TDDS candidate: dose ≤ ~10–20 mg/day, molecular weight below about 500 Da, log P roughly 1–3, melting point under ~200 °C, non-irritant, potent. Marketed examples: nicotine, nitroglycerin, fentanyl, scopolamine, estradiol, rivastigmine, diclofenac.
Permeation enhancers (ethanol, propylene glycol, terpenes, Azone, surfactants) reversibly disorder lipid packing; physical enhancement uses iontophoresis (small current drives ions), sonophoresis, electroporation and microneedles (solid, coated, hollow, dissolving).
Mucosal routes: buccal/sublingual films and mucoadhesive patches (avoid first pass, fast onset), nasal (large vascular surface, nose-to-brain potential), pulmonary (MDI, DPI, nebuliser - respirable fraction needs aerodynamic diameter ~1–5 µm), rectal, vaginal rings, and ocular (Ocusert-type inserts, in-situ gelling drops, contact lenses).
Parenteral depots and implants: oily solutions and aqueous suspensions of poorly soluble esters (e.g. medroxyprogesterone acetate, haloperidol decanoate), PLGA microspheres (leuprolide, octreotide), non-biodegradable implants (etonogestrel rod; Norplant-type levonorgestrel), biodegradable wafers, and in-situ forming gels (PLGA in NMP, poloxamer thermogels).
Sterility and endotoxin limits apply unchanged to every injectable NDDS; terminal sterilisation often impossible for liposomes, so aseptic processing plus sterile filtration (0.22 µm) is used.

Vesicular and particulate carriers (nano/micro)

Liposomes: concentric phospholipid bilayers with aqueous compartments; carry both hydrophilic (core) and lipophilic (bilayer) drugs. Classified by size/lamellarity - SUV, LUV, MLV. Prepared by thin-film hydration, ethanol/ether injection, reverse-phase evaporation, then sized by extrusion or sonication. Stealth (PEGylated) liposomes avoid opsonisation and prolong circulation. Products: amphotericin B liposomal (reduced nephrotoxicity), doxorubicin liposomal.
Niosomes: non-ionic surfactant (Span/Tween) + cholesterol vesicles - cheaper and chemically more stable than liposomes. Transfersomes (ultradeformable, edge activator), ethosomes (high ethanol) and phytosomes are variants for skin delivery.
Polymeric nanoparticles (10–1,000 nm): nanospheres (matrix) and nanocapsules (shell); PLGA, PLA, chitosan, albumin. Made by emulsification–solvent evaporation, nanoprecipitation, salting out, ionic gelation, supercritical fluid.
Solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC); self-emulsifying systems (SEDDS/SMEDDS) and nanoemulsions for BCS class II solubility problems; nanosuspensions (media milling, high-pressure homogenisation) raise dissolution rate via increased surface area (Noyes–Whitney) and raise saturation solubility at small particle curvature (Ostwald–Freundlich).
Microspheres and microcapsules (1–1,000 µm): by coacervation–phase separation, solvent evaporation, spray drying, ionotropic gelation (alginate–calcium); mucoadhesive, magnetic and bioerodible variants.
Dendrimers (hyperbranched, generation-wise, high surface functionality), polymeric micelles (above CMC, core–shell), carbon nanotubes, cyclodextrin inclusion complexes (β-CD, HP-β-CD - solubility and stability).
Targeting: passive - the EPR (enhanced permeability and retention) effect of leaky tumour vasculature, and size-dependent organ uptake; active - ligands such as folate, transferrin, antibodies, sugars (ASGP receptor), peptides; physical - magnetic fields, ultrasound, light, temperature-sensitive polymers (PNIPAM, lower critical solution temperature). RES/macrophage uptake of large or hydrophobic particles is the chief obstacle.
Characterisation: particle size and PDI by dynamic light scattering, zeta potential (|ζ| > ~30 mV suggests good electrostatic stability), TEM/SEM/AFM morphology, entrapment efficiency, DSC/XRD for crystallinity, in-vitro release by dialysis bag or Franz cell.

Biotechnological, regulatory and evaluation aspects

Protein and peptide delivery: oral route defeated by proteases and poor permeability, so depots, pulmonary, nasal, PEGylation, and permeation enhancers are used. Gene/nucleic acid delivery uses viral vectors or non-viral lipoplexes, polyplexes and lipid nanoparticles (LNPs) - the platform behind mRNA vaccines.
Dissolution testing (IP/USP apparatus): I basket, II paddle, III reciprocating cylinder (useful for extended release), IV flow-through cell, V paddle-over-disc, VI cylinder, VII reciprocating holder - V to VII are the transdermal/implant apparatuses. Modified-release products are tested at three or more time points: an early point to rule out dose dumping, a mid point, and a final point for completeness, with specifications as a range.
IVIVC (in vitro–in vivo correlation): Level A (point-to-point, the only one allowing a biowaiver for changes), Level B (statistical moments), Level C (single-point). A validated Level A correlation can support SUPAC-type changes.
Bioequivalence: 90% confidence interval of the test/reference ratio for AUC and Cmax within 80–125%; modified-release products need fasting and fed studies and must show absence of dose dumping with food/alcohol. Indian studies run under CDSCO approval, ICMR National Ethical Guidelines and the New Drugs and Clinical Trials Rules, 2019, with BA/BE centres registered with CDSCO.
Indian regulatory framework: Drugs and Cosmetics Act, 1940 and Rules, 1945; a modified-release version of an approved drug is treated as a new drug (new dosage form/claim) requiring CDSCO (CDSCO-DCGI) approval; manufacture under Schedule M GMP (revised and notified in 2023, with phased compliance timelines for smaller units - the current cut-off dates should be checked as they have been extended); Schedule Y was largely superseded by the 2019 Rules. Indian Pharmacopoeia 2026 (10th edition, in effect from 1 July 2026) is the official compendium; nanomedicine development follows the CDSCO/DBT Guidelines for Evaluation of Nanopharmaceuticals in India (2019). GST on most formulations is 5% since the 22 September 2025 rate revision.
Advantages overall: fewer doses and better adherence, flatter plasma profile, lower incidence of dose-related side effects, site-specific action, possible dose reduction, patent-life extension and product differentiation.
Limitations: higher cost and complex manufacture, dose dumping risk, no easy dose retrieval or titration, food and pH effects, polymer and excipient variability, difficult scale-up and sterilisation of nanocarriers, physical instability (aggregation, drug leakage, lipid oxidation), and regulatory burden of in-vitro/in-vivo correlation.

Key points

NDDS - formulation controlling rate, time or site of release rather than the amount only.
Zero order - constant amount per unit time; the therapeutic ideal; given by reservoir and osmotic systems.
First order - rate proportional to remaining drug; typical of simple matrix and conventional products.
Higuchi model - diffusion from a matrix; cumulative release is linear with √t.
Korsmeyer–Peppas n - release exponent: n ≈ 0.45 Fickian diffusion, 0.45 < n < 0.89 anomalous (non-Fickian), n ≈ 0.89 Case-II (zero-order erosion/relaxation), n > 0.89 super Case-II.
Hixson–Crowell - cube-root law, release governed by change in surface area/erosion.
Ideal CR candidate - t½ ≈ 2–8 h, dose < ~500 mg, soluble, absorbed along the whole gut, wide therapeutic index.
Dose dumping - sudden release of the whole reserve dose; the key safety failure of reservoir and alcohol-sensitive matrices.
Osmotic pump - semipermeable membrane + osmogen + orifice; release independent of pH and motility.
Gastroretention - floating, swelling, high-density or mucoadhesive designs to hold the unit in the stomach.
TDDS - avoids first pass; needs potent drug, MW < ~500 Da, log P ~1–3; stratum corneum is the barrier.
Liposome vs niosome - phospholipid bilayer vs non-ionic surfactant bilayer; both with cholesterol as a rigidifier.
Stealth/PEGylation - hydrophilic corona reduces opsonisation and RES clearance, extending circulation time.
EPR effect - passive tumour accumulation of 10–200 nm carriers through leaky vasculature and poor lymphatic drainage.
Zeta potential - surface charge indicator; magnitude above ~30 mV implies electrostatic colloidal stability.
Apparatus V, VI and VII - the official dissolution apparatuses for transdermal patches and similar systems.
IVIVC Level A - point-to-point in vitro/in vivo relationship; the only level supporting a dissolution-based biowaiver.
Indian regulation - CDSCO under the Drugs and Cosmetics Act, 1940, New Drugs and Clinical Trials Rules, 2019, Schedule M GMP, IP 2026.

Formulas and examples

Zero order: Qt = Q0 − k0·t — Qt = amount remaining (mg), k0 = zero-order rate constant (mg/h)
First order: log C = log C0 − k·t/2.303 — C in mg or %, k in h⁻¹
Higuchi: Q = kH·√t — Q = cumulative amount released per unit area (mg/cm²), kH in mg·cm⁻²·h⁻¹ᐟ²
Korsmeyer–Peppas: Mt/M∞ = k·tⁿ, i.e. log(Mt/M∞) = log k + n·log t — n = release exponent (dimensionless)
Hixson–Crowell: W0^1/3 − Wt^1/3 = ks·t — W in mg, ks in mg^1/3·h⁻¹
Fick's first law (membrane): J = D·K·ΔC / h — J = flux (mg·cm⁻²·h⁻¹), D = diffusivity (cm²/h), K = partition coefficient, h = membrane thickness (cm)
Permeability: J = Kp·ΔC, Kp = D·K/h (cm/h); steady-state rate = Kp·A·ΔC (A in cm²)
Osmotic pump: dM/dt = (A·Lp·σ·Δπ / h)·C — A = membrane area (cm²), Lp = hydraulic permeability, σ = reflection coefficient, Δπ = osmotic pressure difference (atm), C = core solution concentration (mg/cm³)
CR dose: total dose Dt = Di + Dm, where maintenance Dm = k0(r)·td and k0(r) = Css·Clt = Css·Vd·ke; corrected loading Di = Dloading − k0(r)·tp
Steady state from a constant-rate system: Css = k0(r) / (Clt) = Infusion rate / (Vd·ke)
Entrapment efficiency (%) = (drug entrapped / total drug added) × 100; Drug loading (%) = (drug entrapped / weight of carrier) × 100

Example 1 - Designing a CR dose from pharmacokinetics. A drug has Css(desired) = 10 µg/mL, Vd = 20 L, t½ = 4 h; the CR tablet must act for 12 h.

ke = 0.693/4 = 0.173 h⁻¹; Clt = Vd·ke = 20 × 0.173 = 3.46 L/h.
Required release rate k0(r) = Css·Clt = 10 µg/mL × 3.46 L/h = 10 mg/L × 3.46 L/h = 34.6 mg/h.
Maintenance dose Dm = 34.6 × 12 = 415.2 mg.
Immediate-release loading dose Di = Css·Vd = 10 mg/L × 20 L = 200 mg.
Total dose = 200 + 415.2 ≈ 615 mg (practically a 600 mg tablet: 200 mg IR layer + 400 mg CR layer). Note the dose exceeds the ~500 mg comfort limit, so the drug is borderline for once-daily CR.

Example 2 - Fitting release data to Higuchi. A matrix tablet releases per cm²: 2.0 mg at 1 h, 4.0 mg at 4 h, 6.0 mg at 9 h, 8.0 mg at 16 h.

√t = 1, 2, 3, 4 → Q/√t = 2.0, 2.0, 2.0, 2.0 mg·cm⁻²·h⁻¹ᐟ², a straight line through the origin.
kH = 2.0 mg·cm⁻²·h⁻¹ᐟ²; release is diffusion-controlled.
Predicted release at 25 h = 2.0 × 5 = 10 mg/cm². Time to release 12 mg/cm² = (12/2)² = 36 h.

Example 3 - Release exponent (Korsmeyer–Peppas). A HPMC matrix releases 20% at 1 h and 56.6% at 4 h.

n = log(56.6/20) / log(4/1) = log(2.83)/log(4) = 0.4518/0.6021 = 0.75.
0.45 < n < 0.89 → anomalous (non-Fickian) transport: diffusion plus polymer swelling/erosion - the expected behaviour of a hydrophilic swellable matrix.

Example 4 - Transdermal patch area. A drug needs 4.8 mg/day systemically; the measured steady-state flux through human skin is 5 µg·cm⁻²·h⁻¹, and the patch delivers with 80% utilisation efficiency.

Required input rate = 4.8 mg/24 h = 200 µg/h.
Area at 100% efficiency = 200/5 = 40 cm²; at 80% efficiency = 40/0.8 = 50 cm².
A 50 cm² patch (about 7 cm × 7 cm) is practical; if the needed area had exceeded ~100 cm² the drug would be a poor TDDS candidate or would need a permeation enhancer.

Example 5 - Entrapment efficiency and loading. 100 mg drug plus 400 mg PLGA give 420 mg of washed microspheres; assay of the wash liquid recovers 25 mg free drug.

Entrapped drug = 100 − 25 = 75 mg → EE = 75/100 × 100 = 75%.
Drug loading = 75/420 × 100 = 17.9% w/w.
A 10 mg dose therefore needs 10/0.179 ≈ 55.9 mg of microspheres.

System	Mechanism	Order of release	Main risk
Hydrophilic matrix (HPMC)	Diffusion + swelling/erosion	Higuchi / anomalous	Alcohol-induced dose dumping
Insoluble matrix (ethyl cellulose)	Pore diffusion	Higuchi, incomplete	Residual drug left in matrix
Reservoir/coated pellets	Membrane diffusion	Zero order	Dose dumping if film cracks
Osmotic pump (OROS)	Osmotic pumping	Zero order	Costly; orifice blockage
Ion-exchange resinate	Ionic exchange	First order	Variation with GI ionic strength

Carrier	Size	Composition	Typical use
Liposome	25 nm–2.5 µm	Phospholipid + cholesterol	Amphotericin B, doxorubicin
Niosome	10 nm–1 µm	Non-ionic surfactant + cholesterol	Topical, cheaper alternative
Polymeric nanoparticle	10–1,000 nm	PLGA, chitosan, albumin	Oncology, nose-to-brain
SLN / NLC	50–1,000 nm	Solid or mixed lipids	Topical, poorly soluble drugs
Microsphere	1–1,000 µm	PLGA, albumin, alginate	Monthly depot injections

Where it is used

Formulation development (F&D) and product development at Sun Pharma, Cipla, Dr. Reddy's, Lupin, Zydus Lifesciences, Torrent, Intas, Glenmark and Alkem - ER tablets, osmotic systems, MUPS capsules and transdermal patches for regulated markets.
Indian products students will recognise: nifedipine and glipizide osmotic tablets, metformin SR and gastroretentive metformin, diclofenac SR, pantoprazole and omeprazole enteric pellets, mesalazine colon-targeted tablets, liposomal amphotericin B, PLGA leuprolide depots and leuprolide/octreotide monthly injections, nicotine and rivastigmine patches.
Analytical and QC roles: dissolution profile development, f1/f2 similarity comparisons, stability studies under ICH/WHO Zone IVb conditions (30 °C/75% RH for India) and release-mechanism model fitting.
Regulatory affairs and IPR: CDSCO new-drug applications for modified-release versions, ANDA/505(b)(2) filings, DMF work, and formulation patents that extend product life cycles.
Academic and translational research: NIPER centres, IITs, ICT Mumbai, Jadavpur, BITS Pilani and CSIR laboratories on nanocarriers, mRNA-LNP vaccines and microneedles, funded by DST, DBT, BIRAC and ICMR.
Veterinary, agrochemical and nutraceutical controlled-release products use the same polymer science (bolus devices, encapsulated flavours, probiotics).

Interview questions

Q: What is the difference between sustained release and controlled release? A: Sustained release simply prolongs release and the rate falls with time; controlled release delivers at a predetermined, reproducible rate - ideally zero order - for a defined period.
Q: Which drug properties disqualify a molecule from once-daily controlled release? A: A very short half-life with a large dose, a half-life already over about 12 hours, absorption confined to a narrow intestinal window, a narrow therapeutic index, or dose-dependent (saturable) kinetics.
Q: Why does an osmotic pump give zero-order release? A: Water influx is driven by a constant osmotic gradient while the core remains a saturated solution, so a constant volume of constant-concentration solution leaves the orifice each hour, independent of pH and agitation.
Q: What is dose dumping and where is it most likely? A: Sudden release of the entire reserve dose; it is most likely in single-unit reservoir systems whose membrane cracks, and in hydrophilic or lipid matrices whose polymer is soluble in alcohol - which is why alcohol-dose-dumping testing is required.
Q: How do you tell whether release is diffusion- or erosion-controlled? A: Fit the data to the Korsmeyer–Peppas equation; n near 0.45 indicates Fickian diffusion, n near 0.89 indicates erosion/relaxation-driven Case-II transport, and values in between mean both operate.
Q: Why is cholesterol added to liposomes and niosomes? A: It inserts between the acyl chains to modulate membrane fluidity and packing, reducing drug leakage and improving physical and chemical stability.
Q: What does PEGylation achieve? A: The hydrophilic PEG corona creates a steric barrier that reduces protein opsonisation and macrophage/RES uptake, so circulation half-life increases - the basis of stealth liposomes.
Q: What is the EPR effect and what are its limits? A: Leaky tumour vasculature and poor lymphatic drainage let nanocarriers of roughly 10–200 nm accumulate passively; it is heterogeneous between tumours and much weaker in humans than in rodent models, so active targeting is often added.
Q: Which dissolution apparatus would you choose for a transdermal patch and why? A: Apparatus V (paddle over disc), VI (cylinder) or VII (reciprocating holder) - they hold the patch with a defined exposed area so flux per cm² can be measured.
Q: Why is the stratum corneum called the rate-limiting barrier, and how is it overcome? A: Its tightly packed keratinised cells with intercellular lipid lamellae resist permeation; it is overcome with chemical enhancers, prodrugs, supersaturation, iontophoresis, sonophoresis or microneedles.
Q: How does a floating tablet remain buoyant? A: Its bulk density stays below gastric fluid density, either through a hydrated gel-forming polymer that traps air or through CO₂ generated by a bicarbonate–acid effervescent couple retained in the gel.
Q: What regulatory route applies in India to a modified-release version of an already approved drug? A: It is a new drug (new dosage form/claim) under the Drugs and Cosmetics Act, 1940 and the New Drugs and Clinical Trials Rules, 2019, needing CDSCO approval with bioequivalence data (fasting and fed) and manufacture under revised Schedule M GMP.

Common mistakes

Using "sustained", "controlled" and "delayed" release interchangeably, or calling an enteric-coated tablet a controlled-release product.
Plotting cumulative percentage released against time and claiming Higuchi kinetics - Higuchi requires a plot against √t, and strictly applies only up to about 60% release.
Forgetting that the Korsmeyer–Peppas exponent is fitted to the first 60% of release, and reading n from the whole curve.
Mixing units in dose calculations: µg/mL and L/h must be reconciled (10 µg/mL = 10 mg/L) before multiplying.
Assuming higher drug loading always means better entrapment efficiency - the two are different ratios with different denominators.
Claiming nanoparticles automatically target tumours; without surface modification most are cleared by the RES within minutes to hours.
Treating a liposome suspension as terminally sterilisable - autoclaving destroys the vesicles, so aseptic processing and 0.22 µm filtration are used.
Quoting Schedule Y for clinical-trial requirements, which the New Drugs and Clinical Trials Rules, 2019 have largely replaced.