The Formulation Science Behind Extended Drug Release

Months-long release is consequently an emergent property of the entire system. It depends on how the active ingredient, excipients, internal structure, manufacturing history, administration method, and biological environment interact over time. A formulation may contain an established biodegradable polymer and still fail to deliver the intended profile if the drug is distributed unevenly, the particle structure changes during scale-up, the depot forms inconsistently after injection, or the test method does not detect a meaningful process shift.^7–9

The active ingredient establishes the practical boundaries of long-acting formulation development. A months-long product must contain enough drug to maintain exposure throughout the intended interval, but the available space is limited by injection volume, suspension concentration, implant dimensions, and the amount of release-controlling material needed to construct a functional dosage form. Increasing drug loading may reduce the quantity of polymer or lipid that must be administered, but it can also change internal structure, drug distribution, and release behavior. The relationship among dose, loading, and duration must therefore be evaluated within the specific formulation rather than treated as a simple proportional calculation.^7,8

Solubility and solid-state properties influence both platform selection and release. A drug may be dissolved in a vehicle, suspended as solid particles, dispersed through a polymer matrix, or held within a reservoir. In some systems, dissolution of the drug is itself a rate-limiting step. In others, the drug dissolves readily after water enters the dosage form, and transport through the surrounding material becomes more important. Low aqueous solubility can help sustain release from certain depots, but it does not by itself ensure a predictable, months-long profile. The size, form, and distribution of the solid drug phase may still affect loading, local concentration, and the rate at which the payload becomes available for transport.^2,7

Stability requirements become more demanding as the intended release interval increases. The active ingredient must survive manufacturing and storage, remain intact within the depot, and retain its intended activity as the surrounding material hydrates, reorganizes, or degrades. Peptides and proteins can be particularly sensitive to organic solvents, shear, temperature changes, water–oil interfaces, adsorption, and the evolving microenvironment within a biodegradable polymer matrix. These stresses can affect encapsulation, conformation, aggregation, and biological activity before the drug reaches the patient or while it remains within the formulation.^4,5

Once the molecule’s constraints are defined, the formulation scientist must determine which physical processes can generate the desired release curve. Water entry is often the first step. Incoming fluid may dissolve the drug, hydrate a polymer, create aqueous channels, or initiate degradation. The dissolved payload can then diffuse through pores, a swollen matrix, or a rate-controlling membrane. In biodegradable systems, cleavage of polymer chains and progressive erosion can alter porosity and transport pathways. In swelling-controlled matrices, solvent penetration and polymer relaxation affect mobility. Osmotic implants use a different principle: water enters through a semipermeable membrane, generates pressure, and drives drug through a defined outlet.^2,7

Several of these mechanisms may operate within the same product. Early release can arise from drug located near the surface or within readily accessible pores. A slower interval may follow as diffusion occurs through a hydrated but still-intact structure. Later, polymer degradation, pore expansion, or loss of structural integrity may become more influential. The relative contribution of each process changes as the dosage form ages, which is why release cannot always be represented as a single, constant rate.^2,7,10

Biodegradable microspheres divide the dose among particles that can be suspended and injected. After administration, water penetrates the particles, the drug dissolves or redistributes, and the payload exits through pathways created by diffusion and progressive changes in the polymer matrix. Product performance depends not only on the behavior of an individual microsphere but also on the consistency of the entire particle population. Variability in size, internal structure, or drug content can broaden or distort the collective release profile.^4,8

Poly(lactic-co-glycolic acid) (PLGA) is widely used in long-acting microspheres because its properties can be varied through molecular weight, lactide:glycolide ratio, end-group chemistry, and molecular architecture. Formulations may use one polymer grade or combine grades with different characteristics. These variables influence hydration, drug–polymer interactions, degradation, and release, but their effects cannot be reduced to universal rules. The same change in polymer properties may produce different outcomes depending on the drug, loading, manufacturing process, particle structure, and release environment.^4,7,8

Drug distribution within the particle is equally important. The active ingredient may be dispersed throughout the matrix, concentrated in discrete domains, or enriched near internal and external interfaces. Drug located near the surface can contribute to early release, while material embedded more deeply may remain dependent on water penetration, diffusion, or structural changes in the polymer. Encapsulation efficiency also affects the total amount of material that must be injected. A low-loading formulation may require more polymer and a larger suspension volume to deliver the intended dose.^7,10

The microsphere manufacturing process creates the structure that governs these events. Emulsion-based processes form droplets that are subsequently hardened through solvent extraction or evaporation. Double-emulsion methods are often used for hydrophilic peptides and proteins, while single-emulsion approaches may be suitable for selected small molecules. Coacervation relies on controlled phase separation, and spray drying creates particles through atomization and rapid removal of solvent. Each method exposes the formulation to a different combination of interfaces, mixing forces, temperatures, and solidification conditions.^4,8

Process choices affect particle-size distribution, surface morphology, internal porosity, pore connectivity, drug distribution, residual solvent, moisture, and payload integrity. A change in mixing intensity may alter initial droplet size. A different solvent-removal rate may change how rapidly the polymer hardens and whether the drug migrates during solidification. Washing and drying can further modify particle structure. As a result, two formulations with similar ingredient lists can exhibit different release behavior if they were manufactured through different process histories.^8–10

Raw-material comparability presents a related challenge. PLGA materials with similar nominal descriptions or certificate values may differ in molecular characteristics that affect processing and performance. A supplier or lot change cannot be evaluated solely by confirming that the replacement material carries the same general polymer designation. Developers need to understand which material attributes are linked to particle formation, degradation, and release in the specific product.^11,12

Peptide and protein microspheres add another layer of complexity. The manufacturing process must encapsulate the molecule while preserving its structure and activity. Exposure to organic solvents, shear, temperature changes, and aqueous–organic interfaces can destabilize the payload. After administration, adsorption to the polymer and changes in the local environment can further affect recovery and activity. Successful development therefore requires simultaneous control of the polymer matrix, the process, and the molecular state of the drug.^4,5

In situ–forming depots are administered in a flowable form and develop their release-controlling structure after injection. In phase-inversion systems, a biodegradable polymer and drug are dissolved or dispersed in a suitable vehicle. Contact with tissue fluid initiates water entry and solvent exchange, causing the polymer to precipitate and form a semisolid or solid depot. The kinetics of this transition influence the structure that develops at the injection site and the amount of drug released before the depot fully consolidates.^7,13

Burst release is a central formulation concern for these systems. Drug may escape rapidly during solvent exchange, particularly if it remains mobile while channels and pores are forming. Drug located near the formulation–tissue interface may also be released before the polymer structure becomes an effective barrier. Polymer concentration, solvent selection, formulation viscosity, drug solubility, additives, and the rate of phase inversion can all influence this process, although their effects vary among formulations.¹³

The structure of an in situ depot is formed under physiological conditions rather than entirely within manufacturing equipment. This provides the practical advantage of needle-based administration but introduces sources of variability that are difficult to observe directly. Depot geometry may depend on how the formulation disperses at the injection site, how quickly solvent and water move across the interface, and how surrounding tissue constrains the material. Development must therefore connect the properties of the pre-injection formulation with the post-injection structure that controls release.^6,13

Preformed implants shift more of that structural control into the manufacturing process. In a matrix implant, the drug is distributed through the polymer body and released through diffusion, swelling, and, for biodegradable materials, changes associated with degradation and erosion. In a reservoir implant, the drug occupies a distinct compartment surrounded by a rate-controlling membrane. Osmotic implants use water entry and pressure to move drug through a defined outlet. These architectures allow the release path to be engineered through geometry, membrane properties, material selection, and drug distribution.²

Biodegradable implants are designed to lose structural integrity over time and may avoid a separate removal procedure. Non-biodegradable implants retain their material and often require removal or replacement, but an accessible device may allow delivery to be stopped by removing it. Neither approach is inherently superior. The choice depends on the treatment duration, payload, desired release mechanism, administration procedure, and need for reversibility.^2,6

Implant manufacture can involve extrusion, molding, compression, or assembly of reservoirs and membranes. Dimensions, mechanical strength, matrix uniformity, and membrane integrity must support both insertion and controlled release. The applicator and insertion procedure are also part of the product design. An implant that performs consistently in vitro but is difficult to place reproducibly would not meet the practical requirements of a long-acting therapy.^2,6

Lipid-based depots include oil solutions and suspensions, liposomes, solid lipid systems, and formulations that reorganize or form structured phases after administration. These systems can retain drug through slow dissolution, partitioning between phases, diffusion through a lipid matrix, or gradual structural change. The relevant mechanism depends on the composition and architecture of the formulation rather than on the presence of lipid alone.³

Nanoparticles can play several roles within extended-release systems. They may carry the drug directly, protect a labile payload, alter local interactions, or function as components of a larger depot. In hybrid systems, particles may be embedded within a hydrogel, polymer matrix, or structured lipid phase. This arrangement can separate functions: the particle can protect or bind the payload, while the surrounding depot limits movement away from the administration site.^1,3

The term nanoparticle does not describe a release mechanism. Nanoscale size does not by itself produce months-long exposure. Performance also depends on composition, surface properties, particle-size distribution, encapsulation, aggregation, physical stability, and interactions with proteins, cells, and tissue. A nanoparticle formulation intended for prolonged local delivery may require a separate retention mechanism to prevent rapid dispersion or clearance from the site.¹⁴

Hybrid systems offer additional design flexibility but introduce more structural levels that must be controlled. Developers may need to characterize the drug within the particle, the particle within the surrounding matrix, and the evolution of both components during release. Manufacturing must reproduce particle properties, loading, matrix formation, and the interaction between the two. This complexity can be justified when the separate elements address different formulation problems, such as protecting a sensitive molecule while also retaining it locally.^3,14

A long dosing interval is only one feature of a successful release profile. The timing and magnitude of exposure throughout that interval are equally important. Initial burst, lag time, maintenance release, and terminal behavior must be evaluated as connected parts of the product rather than as isolated measurements.

Initial burst commonly reflects drug that is accessible before the main release-controlling structure becomes fully effective. In microspheres, that fraction may be located near the surface or within connected pores. In an in situ–forming depot, drug may escape during solvent exchange and polymer precipitation. In an implant, early diffusion may be influenced by the initial distribution of drug and the hydration of the matrix or membrane. Some initial exposure may support rapid onset, but excessive release can create a peak that is inconsistent with the intended profile.^10,13

A lag phase may occur when water must penetrate the dosage form, the drug must dissolve, or the polymer must swell or degrade before additional transport pathways develop. Whether a lag is acceptable depends on the therapeutic context. A product intended to replace an existing therapy immediately may require sufficient early exposure, while another may be administered with an oral or injectable loading strategy. The formulation must be designed around the intended clinical sequence rather than optimized for a mathematically smooth curve in isolation.

During the maintenance phase, the dosage form is continually changing. Drug concentration falls, polymer molecular weight may decline, pores expand or connect, and diffusion distances shift. The formulation must compensate for these changes well enough to maintain the intended exposure. A structure that releases appropriately during its first weeks may behave differently as degradation becomes more extensive or the remaining payload becomes concentrated in less accessible regions.^7,10

Late-stage release also requires attention. Accelerated erosion, structural collapse, or opening of new pathways can change the release rate near the end of the dosing interval. Conversely, a residual fraction may remain trapped if the structure does not evolve as expected. Characterizing only the early and middle portions of the profile can therefore conceal problems that emerge after prolonged residence.^2,7

Loss of release control, including dose dumping, should be treated as a failure of the intended product design. Prevention depends on material consistency, structural integrity, process control, and analytical methods that can identify changes associated with altered release. Claims about the frequency or cause of dose dumping must remain product-specific, but the broader development principle is clear: duration cannot be separated from the shape and stability of the entire curve.

A long-acting formulation must be deliverable as well as pharmacologically effective. Microsphere and particle suspensions may settle during storage or after reconstitution. They must redisperse sufficiently to provide a uniform dose and pass through the selected needle without unacceptable aggregation or blockage. The time between preparation and administration, the mixing method, and the properties of the diluent can all affect delivery performance.^4,6

Viscosity creates another trade-off. Increasing polymer concentration, particle concentration, or drug loading may reduce the required injection volume, but it can also increase injection force. An in situ–forming formulation must remain flowable before administration while still producing a stable depot afterward. Testing should use the intended syringe, needle, injection rate, and preparation procedure because a formulation’s behavior cannot be separated from the system used to deliver it.⁶

Manufacturing determines whether the release-controlling structure can be produced reproducibly. Critical material attributes may include polymer molecular-weight distribution, monomer composition, end-group chemistry, drug particle size, solid state, lipid composition, and solvent or excipient quality. Critical process parameters vary by platform but can include mixing energy, phase ratios, addition rate, temperature, solvent extraction, particle hardening, washing, drying, extrusion, molding, filling, and assembly.^4,7,8

Scale-up changes more than batch volume. Equipment geometry can alter mixing, droplet formation, heat transfer, mass transfer, solvent removal, and solidification. A larger process may reproduce the same formulation composition while producing particles or implants with different internal structures. Scale-up studies must therefore show that the commercial process recreates the material attributes and microstructure associated with the intended release profile.^8,10

Technology transfer presents the same challenge. A receiving site needs more than a formula and a sequence of operating steps. It needs an understanding of which material and process variables affect particle formation, depot behavior, implant structure, and release. That knowledge supports rational adjustment when equipment, scale, raw materials, or facility constraints differ from those used during development.

The control strategy should connect measurable product attributes with performance. Depending on the platform, relevant endpoints may include particle-size distribution, morphology, porosity, drug distribution, loading, encapsulation efficiency, residual solvent, moisture, polymer properties, membrane integrity, in vitro release, and device-delivery performance. No single measurement establishes equivalence. The objective is to build a set of controls that collectively demonstrate that the process continues to create the intended release-controlling structure.^7,9,10

This need for integration gives contract development and manufacturing organizations (CDMOs) a significant role in long-acting product development. Formulation science, process engineering, analytical development, sterile-product operations, device expertise, scale-up, and technology transfer must be coordinated early. Specialized capabilities may also be required for solvent handling, particle manufacture, implant processing, aseptic operations, and structural characterization. Bringing these disciplines together before the formulation is locked can reduce the risk that a promising laboratory concept proves difficult to manufacture or administer.

The defining performance characteristic of a long-acting formulation may unfold over weeks or months, creating an immediate analytical challenge. Real-time release tests can slow formulation screening, process optimization, stability assessment, and investigations of material or manufacturing changes. Developers need methods that are sensitive enough to distinguish meaningful differences while remaining practical for development and quality control.^9,11

Accelerated tests attempt to shorten the observation period by changing conditions, such as temperature, medium composition, pH, or agitation. Acceleration must be applied carefully. A faster test is useful only if it preserves sensitivity to the structural and formulation differences that matter under physiological conditions. Conditions that force rapid release through an artificial mechanism may conceal differences in polymer degradation, pore development, drug distribution, or membrane transport.

A discriminatory method should detect changes associated with raw materials, particle size, internal structure, loading, manufacturing process, and storage. It does not need to reproduce every aspect of the injection site to provide value, but its purpose must be defined clearly. A method that detects process differences is not automatically capable of predicting clinical performance.^9,14

Establishing an in vitro–in vivo correlation (IVIVC) is especially difficult for complex long-acting products. Multiple mechanisms may contribute to release, the dosage form evolves over time, and the local biological environment is difficult to reproduce in a laboratory vessel. Clinical pharmacokinetic studies may also extend over long intervals, limiting the speed with which formulation changes can be evaluated. An IVIVC must therefore be demonstrated with appropriate evidence rather than assumed from similarity between an in vitro curve and a plasma profile.^4,11

Bioequivalence assessment presents related problems. A comparative product may need to match an early burst, a possible lag, the maintenance phase, and the terminal portion of the profile. Similar active and inactive ingredients do not establish equivalent microstructure, and equivalent average exposure may not capture differences in the timing of release. These challenges explain why regulators continue to study material attributes, manufacturing variables, dissolution methods, modeling, and relationships between in vitro and in vivo performance for long-acting injectables and implants.^9,12,14

Platform selection should begin with the dose and exposure profile rather than with a preferred technology. Developers must determine how much drug must be delivered, how much carrier material can be administered, and whether the required loading is compatible with the intended particle, depot, or implant. They must also evaluate what the molecule can tolerate during solvent exposure, mixing, drying, sterilization, storage, and prolonged residence within the dosage form.

The desired release mechanism should then be matched to the clinical objective. A poorly soluble drug may be suitable for a suspension depot. A molecule that can be dispersed within a biodegradable matrix may support a microsphere or implant approach. A drug requiring membrane-controlled output may favor a reservoir architecture. A sensitive payload may require a hybrid system that separates protection from local retention. The preferred route and administration setting further narrow the options by imposing constraints on volume, viscosity, needle compatibility, reconstitution, insertion, and removal.

Manufacturability and testability must be considered before a platform is selected. A formulation that reaches the desired duration in a small laboratory study may still be unsuitable if its structure cannot be reproduced at scale, its raw materials cannot be controlled adequately, or its release profile requires an impractically long test for every development decision. The strongest platform is the one that combines the required exposure with a controllable process, a practical delivery system, and an analytical strategy capable of monitoring performance.

Future progress will depend on improved understanding of the connections among materials, structure, and release. Average particle size and total loading provide useful information, but they may not reveal differences in internal porosity, pore connectivity, drug distribution, or polymer evolution. More detailed structural characterization can help identify which attributes govern burst, maintenance, and terminal release.

Mechanistic and computational models may further support development by integrating transport, degradation, and material properties. Their immediate value lies in testing hypotheses, organizing experimental data, and guiding formulation studies rather than replacing in vivo evidence. Quality-by-design approaches can provide a framework for defining the target release profile, identifying the variables likely to affect it, and establishing links among process inputs, dosage-form structure, and performance.

Hybrid delivery systems and closer formulation–device integration will continue to broaden the design space. Combining particles, hydrogels, lipids, and polymers may allow different components to address payload protection, local retention, and release control. At the same time, every additional structural level creates new requirements for manufacturing and characterization. Formulations, containers, needles, applicators, and insertion systems must therefore be developed as connected parts of the same product.

Months-long delivery is achieved by engineering a dosage form that changes in a controlled manner. Polymers, lipids, microspheres, implants, and nanoparticles provide distinct tools, but the intended interval emerges only when the molecule, material, structure, process, device, and test strategy function together. The objective is not simply to slow the movement of drug. It is to create a reproducible exposure profile that remains controlled from manufacture and storage through administration and residence in the body.

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2. Stewart, Sarah A, et al. “Implantable Polymeric Drug Delivery Devices: Classification, Manufacture, Materials, and Clinical Applications.” Polymers. 10: 1379 (2018).

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10. Wang, Mengdi, et al. “Microstructure Formation and Characterization of Long-Acting Injectable Microspheres: The Gateway to Fully Controlled Drug Release Pattern.” International Journal of Nanomedicine. 19: 1571–1595 (2024).

11. FY2016 Regulatory Science Report: Long-Acting Injectable Formulations. U.S. Food and Drug Administration. 28 Mar. 2017.

12. FY2018 Regulatory Science Report: Long Acting Injectables and Implants. U.S. Food and Drug Administration. 2018.

13. Bakhrushina, Elena O, et al. “Burst Release from In Situ Forming PLGA-Based Implants: 12 Effectors and Ways of Correction.” Pharmaceutics. 16: 115 (2024).

14. “FY 2025 Generic Drug Science and Research Initiatives Public Workshop: Session 2 Transcript.” U.S. Food and Drug Administration. 6 Aug. 2025.