Regulatory Classification of
Pharmaceutical Co-Crystals
Guidance for Industry

U.S. Department of Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
February 2018
Pharmaceutical Quality/CMC
Revision 1

Regulatory Classification of
Pharmaceutical Co-Crystals
Guidance for Industry
Additional copies are available from:
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Food and Drug Administration
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Email: druginfo@fda.hhs.gov
http://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm

U.S. Department of Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
February 2018
Pharmaceutical Quality/CMC
Revision 1

Contains Nonbinding Recommendations
TABLE OF CONTENTSI.

INTRODUCTION............................................................................................................. 1II.

BACKGROUND ............................................................................................................... 1

III.

DISCUSSION .................................................................................................................... 2

GLOSSARY .................................................................................................................................. 4

Contains Nonbinding Recommendations

Regulatory Classification of Pharmaceutical Co-Crystals
Guidance for Industry 1

This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) on
this topic. It does not establish any rights for any person and is not binding on FDA or the public. You
can use an alternative approach if it satisfies the requirements of the applicable statutes and regulations.
To discuss an alternative approach, contact the FDA office responsible for this guidance as listed on thetitle page.

I.

INTRODUCTION

This guidance provides applicants planning to submit new drug applications (NDAs) andabbreviated new drug applications (ANDAs) with information on the appropriate regulatoryclassification of pharmaceutical co-crystal solid-state forms. 2 This guidance also provides
information about the data that applicants should submit to support the appropriate classificationof a co-crystal as well as the regulatory implications of the classification.
The recommendations in this guidance apply to materials that the Agency has not previously
evaluated and determined to be pharmaceutical co-crystals. The recommendations do not apply
to materials that the Agency has previously designated as salts, complexes, or other non-cocrystalline forms.
In general, FDA’s guidance documents do not establish legally enforceable responsibilities.
Instead, guidances describe the Agency’s current thinking on a topic and should be viewed only
as recommendations, unless specific regulatory or statutory requirements are cited. The use of
the word should in Agency guidances means that something is suggested or recommended, but
not required.
II.

BACKGROUND

Co-crystals are crystalline materials composed of two or more different molecules, typically
active pharmaceutical ingredient (API) and co-crystal formers (“coformers”), in the same crystal
lattice. Pharmaceutical co-crystals have provided opportunities for engineering solid-state

1This guidance has been prepared by the Office of Pharmaceutical Quality in the Center for Drug Evaluation andResearch at the Food and Drug Administration.
2
This guidance finalizes the August 2016 revised draft guidance for industry Regulatory Classification of
Pharmaceutical Co- Crystals, which classified co-crystals as a drug product intermediate (or as an in-processmaterial), and replaces the 2013 guidance of the same name.

1Contains Nonbinding Recommendations
forms beyond conventional solid-state forms of an API, such as salts and polymorphs. 3 Cocrystals can be tailored to enhance drug product bioavailability and stability and to enhance theprocessability of APIs during drug product manufacture. 4 Another advantage of co-crystals isthat they generate a diverse array of solid-state forms for APIs that lack ionizable functionalgroups, which is a prerequisite for salt formation.
III.

DISCUSSION

Co-crystals are distinguished from salts because unlike salts, the components that co-exist in theco-crystal lattice with a defined stoichiometry interact nonionically. In addition, co-crystals
differ from polymorphs, which are defined as including 1) single-component crystalline forms
that have different arrangements or conformations of the molecules in the crystal lattice,2) amorphous forms, and 3) multicomponent phases such as solvate and hydrate forms. 5
Instead, co-crystals are more similar to solvates, in that both contain more than one component
in the lattice. From a physical chemistry and regulatory perspective, co-crystals can be viewed
as a special case of solvates and hydrates, wherein the second component, the coformer, is not asolvent (including water), and is typically nonvolatile.6
For NDAs and ANDAs containing or claiming to contain a co-crystal form, applicants should
submit appropriate data that support the following:•Provide evidence to demonstrate that both the API and coformers are present in the unit cell.

•If both API and coformer have ionizable functional groups, a conclusion that the component
API and coformer co-exist in the co-crystal which interact nonionically. Consider thefollowing to guide your decision:o Generally speaking, if the API and its coformer have a ΔpKa (pKa (conjugate acid ofbase) - pKa (acid)) > 1, there will be substantial proton transfer resulting in ionizationand potential formation of a salt as opposed to a co-crystal. On the other hand, if theAPI and its coformer have a ΔpKa (pKa (conjugate acid of base) - pKa (acid)) < 1,there will be less than substantial proton transfer. If this criterion is met, the APIcoformer entity should be classified as a co-crystal.

3

For the purposes of this guidance, the term active pharmaceutical ingredient is synonymous with the term drug
substance (as defined in 21 CFR 314.3).
4Schultheiss, N and A Newman, 2009, Pharmaceutical Cocrystals and Their Physicochemical Properties, Crystal
Growth & Design, 9(6):2950–2967; Shan, N and MJ Zaworotko, 2008, The Role of Cocrystals in Pharmaceutical
Science, Drug Discovery Today, 13(9–10):440–446.
5See the guidance for industry ICH Q6A: Specifications: Test Procedures and Acceptance Criteria for New Drug
Substances and New Drug Products: Chemical Substances. We update guidances periodically. To make sure you
have the most recent version of a guidance, check the FDA Drugs guidance web page at
http://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm.
6Note that USP infrared identity test <197> may not be applicable when comparing co-crystals with different
coformers.

2

Contains Nonbinding Recommendations
o If, however, it is determined that the classification of the pharmaceutical solid as a salt
or co-crystal is not predicated on these relative pKa values, use spectroscopic tools
and other orthogonal approaches to provide evidence to the contrary.
•Assurance that substantial dissociation of the API from its co-crystal form occurs before
reaching the site of pharmacological activity. Given that the interaction of the API with its
coformer is of similar magnitude to the interaction of the API with solvents in solvates, anin vitro evaluation based on dissolution and/or solubility is generally considered sufficient
to demonstrate that the API dissociates from its coformer before reaching the site ofpharmacological activity.

A co-crystal with a pharmaceutically acceptable coformer that meets the above conditions canbe considered to be a pharmaceutical co-crystal and has a regulatory classification similar tothat of a polymorph of the API. 7 Specifically, it is not regarded as a new API. 8 From a
regulatory perspective, drug products that are designed to contain a new co-crystal areconsidered analogous to a new polymorph of the API. A co-crystal that is composed of two or
more APIs (with or without additional inactive coformers) will be treated as a fixed-dose
combination product and not a new single API.
If you are using a material that the Agency previously considered to be a co-crystal, you may
continue to do so. In new applications formatted as common technical documents, provide
evidence of the previous co-crystal designation in section 3.2.S.1 (Components of the DrugSubstance). 9
The type and extent of characterization and release testing performed on the co-crystal should besufficient to ensure the identity, strength, quality, and purity of the API(s).

7See the guidance for industry ANDAs: Pharmaceutical Solid Polymorphism.
Different co-crystals of a salt API will be treated as a polymorph of that salt.
9For information on common technical documents, see
http://www.fda.gov/drugs/developmentapprovalprocess/formssubmissionrequirements/electronicsubmissions/ucm153574.htm.
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Contains Nonbinding Recommendations
GLOSSARY
Co-crystals: Crystalline materials composed of two or more different molecules, one of whichis the API, in a defined stoichiometric ratio within the same crystal lattice that are associated bynonionic and noncovalent bonds.
Coformer: A component that interacts nonionically with the API in the crystal lattice, that is
not a solvent (including water), and is typically nonvolatile.
Polymorphs: Different crystalline forms of the same API. This may include solvation or
hydration products (also known as pseudopolymorphs) and amorphous forms. Per the current
regulatory scheme, different polymorphic forms are considered the same APIs. (See guidance
for industry ANDAs: Pharmaceutical Solid Polymorphism.)
Salts: Any of numerous compounds that result from replacement of part or all of the acidhydrogen of an acid by a metal or a radical acting like a metal: an ionic or electrovalent
crystalline compound. Per the current regulatory scheme, different salt forms of the sameactive moiety are considered different APIs. (See 21 CFR 314.108 and 21 CFR 320.1(c).)

4