Co-crystals, Salts or Mixtures of Both? The Case of Tenofovir Alafenamide Fumarates

Abstract

Tenofovir alafenamide fumarate (TAF) is the newest prodrug of tenofovir that constitutes several drug products used for the treatment of HIV/AIDS. Although the solid-state properties of its predecessor tenofovir disoproxil fumarate have been investigated and described in the literature, there are no data in the scientific literature on the solid state properties of TAF. In our report, we describe the preparation of two novel polymorphs II and III of tenofovir alafenamide monofumarate (TA MF2 and TA MF3). The solid-state structure of these compounds was investigated in parallel to the previously known tenofovir alafenamide monofumarate form I (TA MF1) and tenofovir alafenamide hemifumarate (TA HF). Interestingly, the single-crystal X-ray diffraction of TA HF revealed that this derivative exists as a co-crystal form. In addition, we prepared a crystalline tenofovir alafenamide free base (TA) and its hydrochloride salt (TA HCl), which enabled us to determine the structure of TA MF derivatives using ¹⁵N-ssNMR (¹⁵N-solid state nuclear magnetic resonance). Surprisingly, we observed that TA MF1 exists as a mixed ionization state complex or pure salt, while TA MF2 and TA MF3 can be obtained as pure co-crystal forms.

Keywords: X-ray diffraction; co-crystal; polymorphism; salt; ssNMR; tenofovir alafenamide fumarate.

Figure 1

Chemical structure of tenofovir and its prodrug fumarate derivatives.

Figure 2

Powder X-ray diffraction patterns of tenofovir alafenamide fumarate derivatives. Red: TA HF; Blue: TA MF1; Green: TA MF2; Magenta: TA MF3.

Figure 3

Thermal ellipsoid figures of (a) TA and (b) TA HF drawn at the 30% probability level. The asymmetric unit of TA HF contains one tenofovir alafenamide molecule and half of a molecule of fumaric acid. The disorder of the phenyl and isopropyl groups in TA HF is omitted for clarity. Hydrogen bonds are drawn with dashed blue lines.

Figure 4

Hydrogen bond architecture in the TA crystal. (a) The interactions involving adenine moieties; (b) Wavy layered structure along the b axis; (c) Packing of layers along the c axis (arbitrary colors). Hydrogen bonds are drawn with dashed blue lines. Hydrogen atoms not involved in the shown motif have been omitted for clarity.

Figure 5

Hydrogen bond architecture in the TA HF co-crystal. (a) Hydrogen bonding between both carboxylic groups of fumaric acid and two adjacent tenofovir alafenamide molecules; (b) Hydrogen bonding between adjacent tenofovir alafenamide molecules and intramolecular hydrogen bonding. Hydrogen bonds are drawn with dashed blue lines. Hydrogen atoms not involved in the shown motif have been omitted for clarity. The disorder of the isopropyl and phenyl groups is omitted for clarity.

Figure 6

¹⁵N CP MAS NMR (¹⁵N cross-polarization magic angle spinning nuclear magnetic resonance) spectra of (a) TA, (b) TA HCl and (c) TA HF.

Figure 7

¹⁵N LG-CP MAS NMR (Lee-Goldburg cross-polarization magic angle spinning nuclear magnetic resonance) spectra of (a) TA and (b) TA HCl. Both spectra were acquired using a relaxation delay of 2 s and ca. 40,000 scans. The relative integral value of protonated purine nitrogen is reported with respect to the integral region of amine nitrogen, which was arbitrarily set at 100.

Figure 8

¹⁵N LG-CP MAS NMR spectra of (a) TA MF1 and (b) TA MF3. Both spectra were acquired using a relaxation delay of 1 s and ca. 200,000 scans. The relative integral value of protonated purine nitrogen is reported with respect to the integral region of amine nitrogen, which was arbitrarily set at 100.

Figure 9

Superposition showing the difference in conformation of tenofovir alafenamide molecules in TA (red) and TA HF (blue). The disorder on the isopropyl and phenyl groups is omitted for clarity.

Figure 10

¹⁵N LG-CP MAS NMR spectra of different samples of TA MF1: (a) the majority of samples contains salt species, (b) the sample contains approximately 30% of salt species and (c) the sample contains approximately 20% of salt species. All spectra were acquired using a relaxation delay of 1 s and ca. 200,000 scans. The relative integral value of protonated purine nitrogen is reported with respect to the integral region of amine nitrogen, which was arbitrarily set at 100.