Administration Routes & Drug Delivery

Absorption enhancers for oral peptide delivery work through several mechanisms: tight junction modulation (transiently opening paracellular pathways — e.g. sodium caprate, chitosan), transcellular permeation enhancement (increasing membrane fluidity or facilitating transcytosis — e.g. SNAC, medium-chain fatty acids), protease inhibition (co-administered enzyme inhibitors that reduce luminal degradation — e.g. aprotinin, bowman-birk inhibitor), and mucoadhesion (prolonging contact time at the absorption site). SNAC's mechanism for oral semaglutide involves both pH buffering (protecting against pepsin) and transcellular absorption promotion in the stomach. The challenge for all absorption enhancers is achieving sufficient enhancement for therapeutic peptide levels without causing mucosal damage from chronic use.

Modern peptide auto-injectors include multi-dose pen devices (FlexPen/FlexTouch for semaglutide Ozempic, with dose dial; KwikPen for dulaglutide Trulicity, single dose per pen) and single-use auto-injectors (spring-loaded devices that automatically insert the needle, inject, and retract). Design considerations: needle gauge (29-32G for minimal pain), injection depth (4-6mm for SC), dose accuracy (±5% for pens), and human factors (usability studies with patients who have limited dexterity, vision, or cognitive function). The pen injector format has been critical to GLP-1 RA commercial success — patient surveys consistently report that pen-based injection is the most preferred delivery method for self-administered peptide drugs.

A bolus injection delivers the entire dose rapidly (typically <1 minute for IV bolus, <30 seconds for SC/IM). This produces a rapid peak concentration (Cmax) followed by distribution and elimination phases. Glucagon emergency treatment for severe hypoglycaemia is administered as a 1mg bolus injection (SC, IM, or IV). Bivalirudin anticoagulation protocol uses an initial IV bolus (0.75mg/kg) followed by continuous infusion. Bolus dosing is appropriate when rapid onset of effect is clinically necessary or when the drug has a short half-life requiring frequent dosing. For peptides with narrow therapeutic windows, bolus administration risks transient supratherapeutic concentrations.

Continuous infusion maintains steady-state drug concentrations without the peak-trough fluctuations of intermittent dosing. Infusion rate at steady state: Rate = CL × Css (where CL is clearance and Css is target concentration). Time to reach steady state during infusion is ~4-5 half-lives (same as intermittent dosing). Octreotide continuous IV infusion (25-50μg/hour) is used for acute variceal bleeding — the constant somatostatin receptor activation suppresses splanchnic blood flow. Bivalirudin continuous infusion (1.75mg/kg/hour) during PCI maintains consistent anticoagulation. Continuous infusion requires IV access and infusion pump, limiting use to inpatient settings.

Depot technologies for peptide drugs include: PLGA microspheres (poly(lactic-co-glycolic acid) — octreotide LAR and some leuprolide formulations use 10-100μm biodegradable microspheres that erode over weeks, releasing peptide; reconstitution technique is critical), PLGA/PLA solid implants (leuprolide Eligard uses in situ forming gel — a polymer solution that solidifies upon SC injection), and self-assembling nanotubes (lanreotide autogel — the peptide itself forms nanotubes at high concentration, creating a viscous gel that slowly releases monomeric peptide). Depot formulations transform daily-injection peptides into monthly or longer-interval treatments, dramatically improving adherence and quality of life.

Enteric polymers (methacrylic acid copolymers — Eudragit L, S; cellulose acetate phthalate; hydroxypropyl methylcellulose phthalate) remain intact at gastric pH (<5) but dissolve at intestinal pH (>5.5-7.0). For oral peptides, enteric coatings protect the compound from gastric acid and pepsin, delivering it to the small intestine where pH is more favourable. However, intestinal delivery still faces challenges from pancreatic proteases and limited epithelial permeability. Interestingly, oral semaglutide (Rybelsus) does NOT use enteric coating — it uses the opposite strategy, designing absorption to occur in the stomach using SNAC technology, because gastric absorption avoids the protease-rich intestinal environment entirely.

Histrelin implant (Vantas for prostate cancer, Supprelin LA for precocious puberty) is a small flexible cylinder (~3cm × 3mm) containing 50mg histrelin in a hydrogel matrix, placed subcutaneously in the upper arm via minor surgical procedure. It provides constant histrelin release for 12 months. Goserelin implant (Zoladex) is a PLGA biodegradable cylinder (~1mm diameter × 1cm length for 1-month, larger for 3-month) injected subcutaneously in the anterior abdominal wall using a large-bore needle device. The PLGA matrix gradually erodes, releasing goserelin. Implants provide the best adherence of any dosing form (100% after placement) and the most consistent drug levels. Disadvantages: need for clinical procedure, potential insertion-site complications, and inability to immediately discontinue treatment.

Systematic rotation prevents: lipohypertrophy (localised fatty tissue growth from repeated insulin/peptide exposure, which can reduce absorption predictability), lipoatrophy (localised fat loss, less common with modern formulations), injection site pain/induration, and nodule formation. Recommended approach: divide each site (abdomen, thigh, upper arm) into quadrants and systematically move through them, with at least 1cm between consecutive injection points. For monthly depot injections (octreotide LAR, lanreotide), alternating between left and right gluteal/SC sites is recommended. Proper rotation education is an important component of patient training for self-administered peptide drugs.

IM injection places drug within skeletal muscle tissue, which has richer blood supply than subcutaneous tissue, potentially allowing faster absorption for some formulations. However, for depot formulations, the IM route provides a large tissue volume for microsphere or gel implantation. Preferred sites: deltoid, vastus lateralis, ventrogluteal, dorsogluteal. Needles are typically 21-23 gauge, 25-38mm. Octreotide LAR (PLGA microspheres) must be administered IM in the gluteal muscle using a specific preparation technique. Leuprolide depot and triptorelin depot formulations are also IM. Healthcare professional administration is generally required for IM injections, which is a disadvantage compared to SC self-injection.

IP injection delivers drug into the peritoneal cavity (~2L volume in humans), where it is absorbed primarily through the portal venous system (draining to the liver) and lymphatics. The large peritoneal surface area (approximately equal to body surface area ~1.7m²) provides extensive absorption surface. In peptide research, IP injection is the most common route in rodent studies due to ease of administration, large volume capacity, and rapid absorption. IP route is less commonly used in human therapeutics but is employed in peritoneal dialysis settings. When reading preclinical peptide research, IP dosing in animal models should not be directly extrapolated to SC or other routes in humans due to different absorption kinetics and first-pass hepatic exposure.

Intrathecal (IT) delivery bypasses the BBB by placing drug directly into the cerebrospinal fluid (CSF) via lumbar puncture or implanted pump/catheter systems. CSF volume is approximately 140mL and is completely exchanged every 6-8 hours. IT delivery achieves high CNS drug concentrations with minimal systemic exposure (reducing peripheral side effects). For peptide therapeutics, IT delivery has been explored for: neuroprotective peptides (targeting CNS pathology directly), analgesic peptides, and enzyme replacement therapies for lysosomal storage diseases. IT administration requires specialised clinical settings and carries risks including infection, headache, and inadvertent neural injury.

IV administration achieves F = 100% by definition (reference standard for bioavailability calculations). IV routes include: bolus (rapid injection over seconds to minutes for acute effect), intermittent infusion (infused over 30-120 minutes at defined intervals), and continuous infusion (constant rate for sustained levels). Bivalirudin is given as an IV bolus followed by continuous infusion during percutaneous coronary intervention. Eptifibatide uses bolus plus infusion for acute coronary syndromes. Carfilzomib is given as a 10-30 minute IV infusion. Difelikefalin is administered IV after each haemodialysis session. IV administration requires venous access and healthcare professional supervision, limiting use to hospitals and clinics.

Microneedle technologies for peptide delivery include: dissolving microneedles (polymer matrix containing peptide that dissolves in dermal interstitial fluid upon insertion, releasing drug — no sharps waste), coated microneedles (drug coated on solid needle surface, deposited upon insertion), and hollow microneedles (miniature hypodermic needles that infuse drug into the dermis). Typical parameters: needle height 200-1500μm, array density 100-2000 needles/cm², tip diameter <20μm. The microneedle approach aims to provide painless, self-administered peptide delivery with room-temperature stability (dry formulations). Phase I/II clinical trials have been conducted for microneedle delivery of PTH, GLP-1 RAs, and other peptides, though none have yet received regulatory approval.

PLGA microsphere manufacturing involves: dissolving the peptide in aqueous solution, emulsifying with PLGA dissolved in organic solvent (oil-in-water or water-in-oil-in-water emulsion), solvent removal (evaporation/extraction), and collection/drying of microspheres. Particle size (typically 10-100μm) is controlled by emulsification parameters. The peptide is entrapped within the polymer matrix and released as PLGA undergoes hydrolytic degradation (ester bond cleavage → lactic and glycolic acid monomers → further degradation to CO2 and H2O). Release kinetics: initial burst (surface peptide), diffusion phase, and degradation-controlled phase. Octreotide LAR microspheres require specific reconstitution technique — the suspension must be prepared and injected promptly to prevent settling and incomplete dosing.

The nasal cavity provides approximately 150-180cm² of mucosal surface area with a rich subepithelial capillary network (respiratory region) and direct access to the CNS (olfactory region). Nasal peptide delivery avoids GI degradation and hepatic first-pass metabolism. Limitations include the mucus layer (which traps particles and peptides), mucociliary clearance (removing deposited material within 15-20 minutes), limited volume per nostril (~100-150μL), and enzymatic degradation by nasal proteases. Approved nasal peptide products: desmopressin spray (10μg/spray for diabetes insipidus — bioavailability ~3-5%), nafarelin spray (200μg/spray for endometriosis/precocious puberty — bioavailability ~2-3%), calcitonin-salmon spray (200 IU/spray for osteoporosis — bioavailability ~3%). Absorption enhancers can improve nasal peptide bioavailability.

The Birmingham Wire Gauge system: higher numbers = thinner needles. Common gauges for peptide injection: 18G (1.27mm OD, used for reconstitution/drawing up), 21-23G (0.82-0.64mm, IM injection), 25-27G (0.51-0.41mm, SC injection with syringes), 29-32G (0.34-0.24mm, SC injection with pen devices). Ultra-thin needles (31-32G) in modern pen injectors significantly reduce injection pain — many patients report minimal or no pain with these devices. Needle length also matters: 4-5mm needles for SC injection in most patients (eliminates need to pinch skin), 6-8mm for patients with thicker SC tissue, 25-38mm for IM injection depending on body habitus and site.

The oral route faces three main barriers: gastric acid/pepsin degradation (pH 1-2 denatures most peptides), intestinal protease degradation (trypsin, chymotrypsin in the lumen; brush border peptidases), and epithelial permeability (tight junctions limit paracellular transport of molecules >600 Da; transcellular transport is hindered by size, hydrophilicity, and lack of specific transporters). Oral semaglutide (Rybelsus) uses SNAC co-formulated in a tablet taken on an empty stomach with ≤120mL water. SNAC creates a localised alkaline microenvironment that protects semaglutide from pepsin, promotes monomeric peptide absorption through the gastric epithelium via transcellular transport, and is itself rapidly absorbed and eliminated. The 14mg oral dose delivers equivalent exposure to approximately 0.5-1mg SC semaglutide, reflecting ~0.4-1% oral bioavailability.

Pen injectors consist of a pen body (housing), a cartridge (containing multi-dose peptide solution or suspension), a dose-setting mechanism (dial), and a disposable pen needle (attached before each injection). Cartridges typically contain 1.5mL or 3mL of drug product. The dose dial allows selection of specific doses — essential for drugs requiring titration (semaglutide: 0.25, 0.5, 1.0, or 2.0mg). After setting the dose, pressing the injection button drives the plunger forward, expelling the dose through the pen needle. Patients are instructed to hold the needle in place for several seconds after injection to ensure complete dose delivery. Modern pens include electronic dose memory features and Bluetooth connectivity for dose tracking apps.

Pre-filled syringes (PFS) are manufactured under aseptic conditions with the drug product already loaded. Advantages: elimination of dosing errors (fixed dose per syringe), reduced preparation time, lower risk of contamination, and improved patient convenience. Glass PFS are most common but polymer (COP) syringes are gaining adoption. Key quality considerations include syringe-drug compatibility (glass surface silicone oil can cause protein aggregation), container closure integrity, and needle sharpness stability over shelf life. For peptide drugs, PFS are used for fixed-dose products where the convenience of single-use, ready-to-inject devices is valued.

SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate, also called salcaprozate sodium) is an N-acylated amino acid derivative developed by Emisphere Technologies. In the Rybelsus tablet, semaglutide and SNAC are co-formulated in a 1:14 ratio (14mg semaglutide with 300mg SNAC). Upon dissolution in the stomach, SNAC creates a localised buffer zone around the tablet remnant that raises local pH (protecting semaglutide from pepsin), promotes semaglutide monomerisation (preventing aggregation that would reduce absorption), and enhances transcellular absorption through the gastric epithelium via a concentration-dependent, transcellular mechanism (not tight junction opening). The fasting requirement (30 minutes before food/other medications) ensures the tablet dissolves in an empty stomach with direct mucosal contact.

SC injection deposits drug into the subcutis (hypodermis) — the adipose tissue layer between the dermis and muscle fascia, typically 1-2.5cm deep. Absorption occurs primarily via blood capillaries and lymphatic vessels. Absorption rate depends on local blood flow (increased by warming/massage, decreased by cold/vasoconstriction), injection volume, and drug formulation. SC bioavailability for peptides is typically 60-90% with Tmax of 1-8 hours. Recommended sites: abdomen (fastest absorption), anterior thigh, upper outer arm. For self-administered peptide drugs, 4-6mm needles at 90° or 8mm at 45° are typical. Modern pen devices (FlexPen, FlexTouch for semaglutide; Trulicity pen for dulaglutide) use 29-32 gauge needles and semi-automated injection to minimise pain and technique errors.

Key sustained release technologies for peptides: PLGA/PLA microspheres (octreotide LAR, leuprolide depot — polymer degradation releases entrapped peptide over weeks to months; release profile depends on polymer molecular weight, copolymer ratio, and microsphere size), in situ forming implants (PLGA in NMP solvent solidifies upon injection — leuprolide Eligard), self-assembling peptide gels (lanreotide autogel — the peptide itself forms nanotubes at high concentration), hydrogel depots, and lipid-based systems. Design goals: zero-order release kinetics (constant rate), complete peptide release, minimal initial burst (avoiding toxic Cmax), and biocompatible degradation products. Manufacturing complexity and cost are significant — depot formulations are typically more expensive than immediate-release equivalents.

Topical delivery targets the application site directly, minimising systemic exposure and side effects. Cyclosporine ophthalmic emulsion (0.05%, Restasis; 0.09%, Cequa) delivers the immunosuppressant directly to the ocular surface for dry eye disease, avoiding the nephrotoxicity and immunosuppression of systemic cyclosporine. Bacitracin ointment provides local antibiotic activity for wound infections. Gramicidin is used in ophthalmic and topical preparations, typically in combination with other antibiotics. For topical peptide delivery, molecular weight, hydrophilicity, and the stratum corneum barrier limit penetration of most peptides to the epidermis or superficial dermis without specialised delivery technologies (liposomes, penetration enhancers, microneedles).

The stratum corneum presents the rate-limiting barrier — its brick-and-mortar structure (corneocytes in a lipid matrix) has permeability limited to small (<500 Da) lipophilic molecules. Since most peptides are large and hydrophilic, passive transdermal delivery is extremely inefficient. Active enhancement technologies include: microneedle arrays (painlessly pierce the stratum corneum, creating micro-channels for peptide diffusion — dissolving microneedles release peptide as they dissolve in dermal fluid), iontophoresis (electrical current drives charged peptide molecules through the skin), sonophoresis (ultrasound temporarily disrupts the stratum corneum), and chemical permeation enhancers. Several transdermal peptide delivery systems are in clinical development but none have reached widespread approval.