Pharmaceutical compounding corresponds to the preparation of unlicensed medicines in order to meet specific patient needs that do not have a licensed (commercial) medicine available on the market. It is no alternative to a licenced orphan drug that follows the usual pathway of development and marketing authorization but compounding is rather complementary if there is no interest from sponsors in the marketing of such a product. More research and development, targeting new pathways and exploring new pharmacological treatment is needed for the designation and authorization of orphan drugs. Patients with rare diseases have individual needs that may benefit from sterile and non-sterile compounded medication since there is a limited number of 144 orphan drugs currently authorised by the European Medicines Agency (EMA). Furthermore, “one-size” does not fit all patients and the existing orphan drugs may not always be adequate regarding the pharmaceutical dosage form or dosage strengths, raw materials (hypersensitivity reactions) or the organoleptic characteristics. For instance, it is common practice to prepare a sildenafil oral liquid instead of dispensing the licensed tablets (Revatio®) for paediatric patients with pulmonary arterial hypertension and the licensed oestradiol (Progynova®) dosage strength often needs adjustments in females with Turner syndrome. Pharmaceutical compounding may also be the only treatment option when the orphan drugs are discontinued or become temporarily unavailable, such as the shortage of ibuprofen injection (Pedea®), the mecasermin solution for subcutaneous injection (Increlex®) in 2013 and arsenic trioxide injection (Trisenox®) in 2017.

In the past, all medicines were compounded formulations prepared secundum artem¹ and it was not until the early 1900s, with the advent of industrialisation and drug manufacturing, that the commercial medicines gradually dominated the pharmaceutical market. At that time, the practice of compounding did not cease to exist but declined substantially worldwide. Nowadays, as we move towards a pharmacogenomics-driven approach to treatment, compounded medication are resurging as an invaluable alternative for optimized treatment outcomes [1] and decreased drug-related adverse effects (intolerance and hypersensitivity reactions). In the United States and Brazil, there are currently compounding-only pharmacies that prepare thousands of compounded medication per day. In the European Union (EU), compounded medication are traditionally prepared extemporaneously in community and hospital pharmacies for one particular patient in accordance with a medical prescription. Notwithstanding this, compounded medication may also be prepared in advance (of a patient’s request), in batches (of variable sizes), and in other settings apart from a pharmacy (e.g. the specials manufacturers in the UK) [2].

The EU pharmaceutical legislation, Directive 2004/27/EC (amending Directive 2001/83/EC on the Community code relating to medicinal products for human use), allows member states to supply compounded medication, namely magistral and officinal formulas, in accordance with the legislation in force and to fulfil special needs. As referred to in Articles 3 [4, 5], a magistral formula is “any medicinal product prepared in a pharmacy in accordance with a medical prescription for an individual patient”; an officinal formula is “any medicinal product which is prepared in a pharmacy in accordance with the prescriptions of a pharmacopoeia and is intended to be supplied directly to patients served by the pharmacy in question”. The EU member states transposed the EU directive and adapted it to their national jurisdictions. As a result, the definition and regulation of pharmaceutical compounding is not harmonized in the EU [6]. For instance, in addition to the EU definition of magistral and officinal formulas, there is the concept of pharmacies’ own formulas in Finland and the specials in the UK [2]. With regards to regulations, third-party compounding (outsourcing of compounded medication) is an example of a practice that is not permitted in Poland, whereas it is a frequent practice in France and Portugal. In some EU Member States pharmacists can eventually outsource all the compounding to other pharmacies or so-called special-order manufacturers (https://www.bnf.org/ page 944–945), especially when the procedure is considered to be too difficult (advanced therapy medicinal products) or dangerous (cytotoxic and radio-active agents and vaccinations).

The quality, safety and efficacy of compounded medication is not guaranteed by randomised clinical trials, as it is for the licensed (commercial) medicines, though health must not be compromised when patients are dispensed both stock and extemporaneous preparations. The EU Resolution CM/Res(2016)1 on quality and safety assurance requirements for medicinal products prepared in pharmacies for the special needs of patients [succeeding Resolution CM/Res(2011)1] highlights the need for quality preparations Europe-wide and sets a list of principles to be adopted by the national frameworks, inter alia an appropriate quality assurance system and a model procedure for risk assessment. As stated, for the “high-risk preparations” it is recommended that the Good Manufacturing Practices (GMP) guidelines [7] are used as reference, whereas for the “low-risk preparations” the PIC/S PE 010–4 guide [8] may be used instead, or other guidelines with an equivalent level of quality [6].

Despite the recommended quality guidelines, compounded medication have endangered public health in unfortunate events worldwide, some with devastating repercussions. Intoxication after percutaneous resorption in babies and small children (skin thickness different from adults) with rare skin diseases has been reported for erroneously prescribed salicylic acid [9, 10], urea [11, 12] and lactic acid [13]. Side-effects have also been published due to an accidental overdose caused by a compounding pharmacy error [14] or by switching from one chemical form to the other [15].

The quality and safety of compounded medication also relies on the active pharmaceutical ingredients and excipients used in the formulations as for authorized orphan drugs. When these are registered pharmaceutical compounds [19] with a monograph in the European Pharmacopoeia (for example creatin, dapsone, glycine, hydroxocobalamine, ketoconazole, l-carnitine, oxybutynin, primaquine phosphate) or any other pharmacopeia (for examples 3,4-diaminopyridine, pyridoxal phosphate) a pharmaceutical analysis of the ingredients can be performed and a certificate of analysis delivered. When there is no monograph yet (for example bi-myconase, cholic acid, l-citrulline, diphencyprone, mepacrine, uridine), the chemical, pharmaceutical and microbiological quality of the ingredients should be demonstrated on the basis of validated methods [6, 20]. A certificate of analysis is an essential document in any compounding procedure.

Compounded medication is now considered as an existing “satisfactory treatment” for whom significant benefit needs to be proven.

Pharmacists already compounded medicinal products for their patients with rare disorders long before they became authorized orphan drugs in the EU [22] such as 3,4-diaminopyridine capsules (Firdapse) [23], 5-aminolevulinic acid drink (Gliolan), betaine drink (Cystadane), caffeine citrate injection (Peyona), cholic acid capsules (Kolbam, Orphacol), cysteamine bitatrate capsules (Cystagon, Procysbi) and eyedrops (Cystadrops) [24], ibuprofen injection (Pedea), p-aminosalicylic acid capsules (Granupas), and zinc acetate capsules (Wilzin). These extemporaneous preparations were made in accordance with an individual medical prescription (“Fac secundum artem”) eventually with a formulation published in a pharmacopeia or according to other references. Several excellent review articles have been published with compounding formulation for the treatment of patients with genodermatosis such as ichthyosis [25].

Sterile [26] and non-sterile compounding for patients with rare diseases is routine pharmaceutical practice today: infusion bags for total parenteral nutrition (neonates with inborn errors of metabolism); medicines and placebos for phase 1 clinical trials with designated orphan drugs; radioactive and other drugs with stability limitations in PK/PD studies for rare disorders; (repurposed) medicaments awaiting orphan drug authorization or withdrawals (drug shortages); masking of taste and smell in medicines for babies with metabolic disorders; and medicinal products for special populations such as paediatrics [27].

The safety and efficacy of most of the treatments with compounded medication are not based on randomised clinical trials establishing the safety and efficacy but on evidence collected during years of clinical practise. Compounded medicinal products are therefore dispensed without a package insert but some pharmacists and/or formularies developed their own package inserts as pharmaceutical care is very important for a better care to the patients. Compound specific databases comprise ChEMBL, ChemSpider, PubChem, OpenPHACTS and the Orange Book with supportive information on a compound’s chemistry and pharmacology.

Drug shortages for orphan (Pedea) and non-orphan drugs (Ammonaps) did appear [28] and compounding was necessary then to continue the life-saving treatment. For other drug shortages (Cerezyme, Fabrazyme) compounding was not possible because the active ingredient was not available and patients didn’t get their proper treatment as long as the shortage took. When a drug is taken from the market by the sponsor (sometimes used off-label), pharmacists had to compound the medicinal product to continue the treatment of their patients with rare disorders. For example Mexitil was taken from the market by the pharmaceutical company and substituted by compounded capsules with mexilitine to continue the successful treatment of patients with myotonic disorders. Fludrocortisone capsules to treat patients with Addison syndrome are not (anymore) on the market in every EU Member State and therefor need to be compounded in some countries. Patients awaiting the marketing and reimbursement of repurposed medicines [29–31] were treated during this process by compounded preparations (propranolol for infantile haemangiomas).

Stability of compounded medicinal products has been published elsewhere (http://www.stabilis.org/) but is always more problematic for medicinal products produced by the pharmaceutical industry as extemporaneous preparations are used almost immediately after production. When intended for injectable use, extensive stability information can be found in the Handbook of Injectable Drugs (http://www.ahfsdruginformation.com/handbook-on-injectable-drugs/).

The cost for the patient is mostly lower for compounded preparations than the rather high cost of an authorized (orphan) drug [32–34]. A European legislation is in place to regulate standardized pharmaceutical compounding [4, 5] but local regulations still need to be installed for other aspects such as the reimbursement of the compounded medicinal products and the authorization for the use of some active and non-active ingredients. Especially for those that are on the WHO Model List of Essential Medicines for Children such as dapsone, fludrocortisone, hydroxocobalamin and primaquine. A Royal Decree in Belgium will regulate this from November 1^st 2017.

A new area in pharmaceutical compounding has arrived with Advanced Therapy Medicinal Products (cell therapy, gene therapy and tissue engineering) with his specific legal framework and Commision-of-Advanced-Therapies classifications. Several EU Member States installed the hospital exception to allow hospital pharmacies to produce these innovative medicines for their hospitalized patients. This will require cooperation with specialized personnel often outside the pharmacy such as blood banks and cell labs. Standardization in human stem cell therapy is also needed. Furthermore a protein bedside-manufacturing model on demand and at a small scale has been described for proteins [35] providing just enough drug for one single patient. This could open new possibilities for ultra-orphan drugs and individualized medicines. Finally 3D printed medicinal products incorporating biomaterials, 3D printed medicines and 3D bioprinting, still experimental and in need of EU legislation (such as biological tissues, organs and cells for medical applications), are compounded today in third line hospitals [36]. Personnel need to be trained and equipment installed to perform these new compounding procedures to obtain medicinal products of the best possible quality.

When there is no on-label or even no off-label treatment for patients with rare diseases pharmacists have to compound the medication. This needs to be done in the best possible conditions by trained compounders following validates procedures.

Desktop research revealed that several validated standard operating procedures for the compounding of medicinal products for patients with rare diseases are already published in different compounding formularies. Medical prescriptions for such a treatment should mention the reference of the formulary for more harmonization as the same medical prescription needs to result in the dispensing of the same compounded medicinal product in all hospital and community pharmacies. Shared pharmacy work documents need to be followed to compound a medicinal product conform to all the quality standards of the European Pharmacopoeia. All ingredients used in the compounding must be accompanied by a certificate of analysis and full records of the pharmaceutical production process need to be kept for full traceability and accountability.

In this way medicinal products of the best possible quality will be dispensed to our patients with rare diseases. “When it’s not in the book, it’s up to the cook”.