Formulation development and manufacturing of multi-active component dosage forms present many unique challenges. These challenges may arise due to following scenarios: 1) physicochemical incompatibility of the active pharmaceutical ingredients (APIs) with each other and with the common excipients used in the FDC formulation; 2) changes in the rate and extent of in-vitro dissolution and in-vivo bioavailability of APIs from FDC as compared to that observed for the single component formulations of individual APIs; 3) undesirable mechanical powder flow and compression characteristics of the multiactive granulation/powder blend; and 4) increase in dosage bulk volume due to additional drug loading particularly for high-dose combinations.
For these reasons, a conventional tablet or capsule dosage form may rarely suffice to meet the various stability, bioequivalency, and commercial manufacturing requirements. Combination of APIs classified as Biopharmaceutics Classification System (BCS) Class II and IV are particularly challenging and require specialized solubility enhanced formulations and amorphous conversion processing technologies that are capable of providing stable, bioavailable, and scalable formulation to accommodate the range of drug loading and potencies of each API. There could be also requirements for individual modification of drug release and pharmacokinetic profiles such that a combination of both immediate and extended drug release formulations may be needed for the combination dosage form.
If an API is initially formulated as a liquid or semi-solid encapsulated composition, its reformulation into a biocomparable solid state formulation could be a prerequisite in order to make the combination with other drugs viable. A comparative in-vivo bioavailability study of liquid versus solid reformulation having a similar food effect can be used to determine if a modest upward potency adjustment can be supported by prior preclinical toxicological safety data to achieve a bioequivalent pharmacokinetic profile for the reformulated product. For BCS Class II and IV compounds, this may involve use of complex solid solubilization co-processing techniques such nanomilling, amorphous API solubilized polymeric dispersions, and surfactant co-granulation technologies.
Multilayer tablet technology is particularly useful for formulating FDC. It allows for compression of separate drug layers, thus minimizing physicochemical incompatibility and stability problems that may arise from the intimate contact of individual drug compositions. Atripla® (Gilead Sciences, Foster City, CA, USA) comprises of a dry granulation layer of tenofovir disoproxil fumarate and emtricitabine and a wet surfactant co-granulation layer of efavirenz . Bilayer tablet technology also provides flexibility in combining formulations with sustained-release and immediate-release layers in the same dosage unit. This was utilized in developing combinations of extended release niacin (controlled-release layer) and immediaterelease layer of laropiprant with simvastatin or atorvastatin (Tredaptive in European Union [EU]). It is noted that physical and mechanical aspects of bilayer tablet compression is more complex than conventional single layer tablet compression. Challenges encountered are due to weight control of the thinner drug layer compression fill weight, and interfacial delamination. The latter may occur as a result of inadequate interfacial bonding strength immediately following ejection or upon further processing stress and attrition such as in pan film coating operation. Delamination may also occur due to differential expansion of the layers upon exposure to elevated humidity conditions upon storage and stability testing.
A strategy to reduce dosage size and enhance swallow ability is to apply precision film coating to deposit API as a polymer film layer (rather than a separate granulation layer) over a separate drug containing tablet core . The drug core could be an extended release matrix core, a conventional tablet core, or a bilayer tablet. These formulation designs provide flexibility for various scenarios of combining multiple APIs, dosing combinations and drug release rates.
FDC development strategies for new chemical entities (NCEs) are increasingly being incorporated early during the compound development program, so that formulation design activities and manufacturing process development for both monotherapy and combination are performed in parallel. This strategy aims to avoid formulation switching and bioavailability issues from occurring late in the development program and commercial stages. However, in many cases FDC development takes place after initial approval of monotherapy is obtained. Therefore, existing knowledge of APIs preformulation and dosage form formulation and stability mechanisms can be heavily leveraged in experimental design to streamline and fast track FDC development and regulatory filing as much as possible.
A further strategy to minimize formulation development particularly when drug potencies are low is to incorporate them in a multiphase capsule technology, which involves co-encapsulation of existing tablet/powder blend/multiparticulates components in potentially sealed compartments. For example, Lotrel® (Novartis, East Hanover, NJ, USA) is marketed as a hard gelatin co-encapsulation of benazepril hydrochloride film-coated tablet and amlodipine besylate granulation powder blend.
FDC Formulation, Manufacturing, and Regulatory Development Roadmap
There are common strategies and approaches that could be taken to successfully develop and market FDC products. The initial project scope and gap analysis activities could be initiated by a review of internal therapeutic area portfolio and by identifying unmet patient needs, new data or indication arising from clinical research and/or clinical practice setting as well as market and brand opportunities. Following gap analysis, the next steps are idea generation, prioritization, and a preliminary review of related API and formulation competitive IP landscape that may depend on combining approved drugs or NCEs. Assessment of risks and probability of success of specific drug combinations and drug delivery technologies are needed in order to arrive at proper recommendation and risk-mitigation plan.
These multidisciplinary plans are used to obtain initial organizational go/no-go decision to allow for allocation of resource funding pending more detailed technical and manufacturing reviews. This could be achieved by conducting a paper feasibility exercise focusing on technical API, formulation and process design aspects as well as regulatory and manufacturing challenges and opportunities for resolution. The paper feasibility assessment may contain several sections from information collected or generated during the prior project scoping and idea generation stages:1) target product profile; 2) API sourcing; 3) preclinical safety pharmacology considerations; 4) FDC formulation and process design; 5) IP issues; and 6) regulatory filing strategy.
Target Product Profile
Target product profile comprises the first step in broadly defining objectives and requirements for meeting product target profile in terms of goals set for disease indication, clinical efficacy, safety, potency, route and frequency of administration, bioequivalencey criteria, and shelf-life stability. These objectives are aligned with organization of sections in the product’s label.
Target product quality profile (TPQP) aims to predefine chemistry, manufacturing and controls (CMC) requirements and performance criteria for product critical quality attributes (CQA) that focuses on guiding formulation, and manufacturing process development efforts. TPQP can be inclusive of predefining drug substance physical form, its biopharmaceutical properties, as well as dosage form characteristics, such as assay, content uniformity limits, compression properties, microbial limits, and impurity/degradant projected specification limits. In addition, dosage form performance criteria such as in vitro-dissolution profile, in-vivo bioavailability profile, stability, and package requirements are included . These critical quality product attributes are refined and revised based on new data that are generated during the later experimental and development stages.
In some situations a FDC product may involve combining an externally sourced API with therapeutic agent/s from innovator’s own internal development portfolio (whether marketed or an NCE). Externally sourced API often presents challenges in terms of its procurement. Such issues and risk-mitigation plans can often be considered in advance during the feasibility assessment stage and may include the following areas: 1) API vendor selection, vendor suitability, and status of prior quality assurance assessment if any, and procurement lead times and cost factors; 2) availability of drug master files for investigational new drug regulatory filing in US, as well as early identification of IP factors that may require sourcing from overseas API vendors; 3) availability of Good Manufacturing Practice (GMP) quality and non-GMP material meeting standard pharmacopeia specifications as needed (US Pharmacopeia [USP], EU, Japanese Pharmacopeia [JP]) for conducting initial feasibility and stability studies; 4) applicability of drug importation requirements, such as securing in advance, investigational new drug application approval from regulatory authority for importation of GMP-quality material for human studies.
FDC Formulation and Process Design
Design of multi-active combination products requires knowledge of each individual API physicochemical and biopharmaceutical properties as well as sources and mechanism/s of chemical degradation and physical instability. API physicochemical profiling data are used to predict associated developability risks, and formulation complexity factors. For BCS Class II and IV compounds, initial selection of preferred solubilization enhancing technologies and inclusion of functional excipients depend on API physico-chemical and gastrointestinal (GI) physiological factors . Traditional high-throughput robotic salt, and polymorph screens are supplemented by solubility screens of lead forms in various pH media, surfactant and vehicles that are intended to inform on solubility and dissolution of API (and its salt) forms as well as API-excipient blends under gastrointestinal relevant conditions [54–56]. The outcome is lead formulations that can achieve dissolution and supersaturation.
Selection of appropriate API phase, salt form and physical state together with API solubility data in physiologically relevant pH media (encountered in fasted stomach and proximal intestine), as well as estimated clinical dose solubility ratio, and intrinsic permeability can be used to forecast whether the oral absorption process is predominately limited by apparent solubility or by dissolution rate. Such predictions are increasingly made using physiologicallybased drug absorption modeling (PBPK), which represents the compartmental absorption and transit model (ACAT) implemented in the commercially available software such as GastroPlus, (Simulations Plus, Lancaster, CA, USA), and PK-Sim (Bayer Technology Services GmbH, Leverkusen, Germany) [57, 58].
Computational absorption simulations particularly when coupled with input parameter sensitivity analysis that for example simulates exposure for a range of doses and particle sizes may point to viable formulation options that are intended to improve oral bioavailability [59, 60]. These may include effect of salt formation, particle size reduction, amorphous and lipid formulations. In addition, in silico methods may be used to aid initial selection of suitable excipients, such as polymer and surfactants for drug solubilization. The latter includes molecular dynamics simulations capable of generating solubility and Flory-Huggins interaction parameters values as a means of predicting drug miscibility in drug-polymer binary mixtures. For solute-solvent pairs, it has been reported that values for difference in solubility parameters of less than 7.5 (J/cm3)1/2 indicate good solubility of solute in the solvent. Furthermore, thermodynamic predictions can provide energetically favorable ranges of API/ polymer drug loading levels that result in either stable solid solution (up to about 10% drug loading) or metastable supersaturated solid solutions (up to approximately 40% DL) [61, 62].
Formulation design requires understanding of the API phase properties and its solubility/miscibility in excipients coupled with the application of appropriate pharmaceutical processing technologies designed to enhance apparent solubility (i.e., maintain a supersaturated state) and stabilize API solid-state transitions.
In-vitro dissolution of weak bases and their salts in simulated gastric and intestine fluid media (FaSSIF and FeSSIF containing bile salts)  is particularly relevant to indicate how dissolution of drug dose in luminal fluid volume is effected by solubility and supersaturation/ precipitation behavior of the free base in the GI pH range covering pre/postprandial conditions [64, 65]. For example, initial incomplete dissolution of very low soluble weak bases (and their salts) in acidic environment of fasting stomach may lead to rapid supersaturation and precipitation of the free base not likely to be dissolved/absorbed in the upper small intestine. Also, poorly soluble weak bases typically show higher rate and extent of in-vitro dissolution in FeSSIF media (as compared to FaSSIF media) indicating the solubilizing effect of bile salts/lecithin present in such media and hence forecast presence of significant food effect on their oral bioavailability profile. On the other hand, sparingly soluble weakly acidic molecules that have high permeability are well absorbed at the intestinal absorption site provided its relevant pKa (carboxylic acids 4–6) lies within a range to permit ionization of the unionized molecule . Several broad categories of excipient that are commonly utilized in various solubilization technologies, such as nanomilling, liquid formulations, and amorphous solid dispersion technologies are given in Table 5 [36, 55, 67–78].
The chemical compatibility of the APIs with each other and with excipients may not be easily predicted from available chemical structure and known degradation pathways. Similarly, consolidation characteristics and flow properties of composite powder and granule blends are not known. Preliminary accelerated stability and compaction simulation design-of-experiment (DOE) studies are flagged for experimental evaluation in the feasibility assessment exercise.
There may be a need to formulate FDC of drugs that are designed to have different diffusion and dissolution kinetics in the same dosage form. Technology approaches used to achieve modified or sustained drug release can vary greatly and may include formulation of multilayer tablets having separate immediate release and modified-release drug layers either as solid granulation layers or as API film layer coated over an uncoated matrix core or over a tablet core with a polymer film coating . Delayed-release, sustained-release and immediate-release multiparticulates alone or in combination have been designed and encapsulated as active containing particulates, or as nonpareil seeds coated with successive API and polymer film layers to impart different drug release characteristics for actives that are either embedded within the particulates or are spray coated as a film.
Pharmaceutical processing technologies needed to manufacture FDC with both immediate and sustained release performance are more complex and the need to conduct well-controlled process characterization studies should be documented in the feasibility assessment plan. Statistical experimental design and implementation of process monitoring and control strategies such as process analytic tools are employed and repeated at different scales to empirically derive the mathematical relationship/s between and amongst critical material and process parameters affecting product quality attributes, and to provide an estimate of process variability. Such DOE studies may eventually lead to determination of the design space of critical material and process parameters that bounds the controlled operating conditions capable of consistently producing product meeting CQA specifications [52, 79]. In other aspects, a range of FDC drug loading levels (based on type of formulation and technology being employed) and required potencies can be used a priori to estimate overall oral dosage unit size, and hence, its ease of swallowing and patient acceptability limitations of dosing single or multiple units.
It is important to analyze the proposed combination therapy against potential competitive therapies and FDC technologies that are in the pipeline, as these competitive technologies may appear well before the proposed FDC reaches the market place. Furthermore, a FDC may combine an innovator’s own therapeutic agent with a generically available active component which may still be subject to unexpired formulation patent or to API physical form patent such as a different hydrate, solvate, or polymorph. In these situations, the paper feasibility exercise should include a preliminary understanding of all applicable IP issues and limitations in both API phase and formulation design in order to streamline development effort.
This can be challenging particularly if generic API of interest has low solubility or has to be formulated as sustained release in the FDC. Such methods of composition and process are usually subject to rich IP protection and patent extensions. These factors are critically reviewed as they may restrict or affect the process route, composition, and technologies that can be employed to design the proposed FDC with its target bioperformance. In particular, a component of regulatory approval relies on establishing bioequivalent performance of the newly formulated FDC with respect to the original reference listed drug (RLD) as defined by FDA in its Orange Book .
Preclinical Safety Pharmacology Considerations
The need for preclinical toxicology data may predicate on having prior co-administration studies, which have demonstrated efficacy and safety of the proposed combination therapy for the target indication in clinical studies of patient populations. Regulatory requirements for conducting preclinical safety pharmacology studies and drug-drug pharmacokinetic interaction studies must be considered for novel combinations even if the individual components are known (for which no concomitant use data exists) and for those involving a new investigational drug.
Before initiating first-in-man clinical investigation of the combination product, it is necessary that the safety profiles of individual APIs are established. The latter includes conventional pharmacology and toxicology studies, such as genotoxicity, mutagenicity, and immunotoxicity. These considerations include establishing maximum-tolerated doses (or in some cases maximum feasible dose) of the drugs alone as well as their combination in GLP (good laboratory practices) acute and repeat dose toxicity studies in preclinical animal species. Maximum-tolerated single dose establishes preliminary safety margin related to proposed human dose. The no observed adverse effect level (NOAEL) demonstrates adequacy of safety margin based on projected exposures in humans at proposed therapeutic range and above milligram per kilogram doses of each active constituent [6, 7].
Clinical Development and Regulatory Strategy
Although several scenarios are possible depending on the type of FDC being developed, a pilot bioavailability study in man is conducted early on in development to evaluate the bioperformance of the combination drugs being formulated into a single unit. The pharmacokinetic bioavailability study is designed as a crossover, open-label, single-dose study in a small number of healthy subjects to determine the comparative bioavailability of the FDC dosage form versus that of the individual active entities co-administered, on separate occasions, at corresponding doses. Subjects are randomly assigned to each of possible sequences with adequate washout period between product administrations. Furthermore, since the purpose of a probe pharmacokinetic study is to assess the probability of success of an eventual definitive bioequivalence study, wider comparative bioavailability (74–143%) instead of bioequivalent (80–125%) acceptance limits may be used. These limits bound the 90% confidence interval of the geometric least square mean ratios (GMR) of both area under curve (AUC0–∞) (μM.hr), and Cmax (nM) of FDC over that of each of its corresponding active components. Statistics pertaining to C24°hr (nM), Tmax (hour) and apparent t1/2 (hour) may be included to inform on the shape of the systemic exposure profiles.
The probe pharmacokinetic study could be designed to include a fed arm in the same study subjects to determine the effect of nutrients on drug absorption and pharmacokinetic endpoints. The fed arm of the study is usually carried out by administering a standard meal (breakfast or evening meal) just prior to dosing. If more than one dose level is being investigated the fed arm may test the worst-case scenario by determining the effect of food/nutrients on pharmacokinetic parameters at only the highest dose. It is noted that this type of fed versus fasted pharmacokinetic study does not constitute a food effect study. A separate food effect study of the FDC may be warranted where the effects of high-fat meal versus low-fat meal on systematic exposure and pharmacokinetic endpoints are evaluated.
For combination products of previously approved drugs, ultimately a sufficiently powered definitive bioequivalence study is required as part of the regulatory filing strategy [80–82]. Drug products are considered therapeutically equivalent if they meet FDA regulatory criteria of pharmaceutical equivalence and bioequivalence. The definitive bioequivalence study uses the optimized to-be-marketed formulation of FDC (usually at highest strength) versus co-administration of the appropriate reference drug products such as innovator’s own approved drugs or in the case of generics, the reference listed drugs at the strength specified in the Orange Book . Chen et al.  proposed that therapeutic equivalence may be established by matching the in vivo drug delivery profile between the reference and the comparator drug products by first identifying critical variables that serve as in vitro markers for characterizing the desired drug delivery profile in vivo.
As an example, an accelerated regulatory filing strategy may consist of a complete phase 1 pharmacokinetic program, pivotal bioequivalent studies plus one or more clinical noninferiority efficacy studies (particularly for EU), wherein bridging to clinical safety and tolerability data in the original new drug application (NDA) submissions are possible. Haider et al.  have proposed a new replicate study design for adjusting the standard bioequivalence limits for drug products for which the coefficient of within subject variability is greater than 30%. This has the advantage of alleviating the need for large number of subjects normally required for traditional bioequivalence trials.
When the FDC product contains one or more API formulated to possess a modified release profile a new NDA submission may generally be required, if this represents the first modified release formulation of the same previously approved immediate-release drug product. However, for any subsequent FDC in which the modified release formulation of an active component is considered to be pharmaceutically equivalent to the marketed controlled-release product (at the specified strength) aforementioned bioavailability and bioequivalent requirements may apply. Furthermore, the bioavailability studies are designed to establish: 1) absence of occurrence of any dose dumping, and 2) consistent pharmacokinetic performance between individual dosage units after both a fasted and fed single-dose study.
For an immediate-release/modified-release FDC (such as a bilayer tablet), an additional complicating factor may arise if no prior clinical data are available for the co-administration of the individual actives formulated as separate immediate release and modified release drug product entities. However, in one scenario, assuming that prior co-administration data does exist for immediate release versions of the actives, a possible resolution could be to provide supportive scientific literature that show non-inferior efficacy and similar safety of known immediate release and modified release product versions of the same API.