Symeres has over 25 years of experience in the synthesis of vitamin D derivatives and intermediates. Throughout this extensive history, we have developed deep expertise in synthesizing reference compounds from the literature, as well as designing both known and novel derivatives for use in diagnostic applications and medicinal chemistry programs. Our capabilities include route scouting and route development for specialized derivatives, enabling us to support a wide range of pharmaceutical and diagnostic needs in vitamin D research and development.
The sensitive nature (towards light, temperature, and pH) and potency of vitamin D derivatives make handling of these compounds challenging. The combination of the unique triene system, multiple chiral centers, and the specifically positioned hydroxyl groups or other functional groups often require lengthy, linear synthesis routes to deliver the desired final analogs. Often, photochemical transformations are part of the synthesis route.
Extensive experience with this type of project ensures that Symeres’ scientists have the knowledge and experience to tackle challenging chemistry.
Over the years, Symeres has synthesized many new derivatives from scratch, including stable deuterium-labeled analogs. Our synthesis routes have proven suitable for scale-up to prepare multigram amounts of these highly potent final compounds, or hundreds of grams of building blocks that can be used for cGMP synthesis.
Some background on this intricate class of compounds is provided below.
Natural synthesis
Vitamin D is a fat-soluble vitamin. Two different primary forms exist, vitamin D2 (ergocalciferol) is derived from plant sources, whereas vitamin D3 (cholecalciferol) is the form that is present in animals and humans. Both share the same characteristic triene moiety, but they differ in the structures of their side chains. Vitamin D is generated from a steroid precursor in the skin upon exposure to sunlight. Through a photochemical reaction driven by UVB radiation, 7-dehydrocholesterol undergoes a sigmatropic rearrangement, which results in breaking of the steroid B-ring, followed by thermal shifting and E/Z isomerization of the conjugated double bonds to provide the characteristic vitamin D structure (Figure 1). Vitamin D is then transported from the skin to other parts of the body, mainly bound to vitamin D binding protein (DBP).
Figure 1: Photochemical production of vitamin D
Despite the difference in the side chains of vitamins D2 and D3, their metabolism and the biological actions of the main metabolites are very similar. Vitamin D undergoes two subsequent hydroxylations to convert it into the active substance. These hydroxylations are mediated by cytochrome P450 (CYP) enzymes. The first hydroxylation occurs in the liver, where 25-hydroxyvitamin D [25(OH)D or calcidiol] is formed; this is the main circulating form of vitamin D. A second hydroxylation in the kidneys converts it into the biologically active form, the hormone 1,25-dihydroxyvitamin D [1,25(OH)D or calcitriol]. Blood levels of calcitriol are usually very low, and it has a short lifetime.
Metabolism
The metabolism of calcitriol is also mediated by CYP enzymes. The first step in the catabolism of the active metabolites is generally 24-hydroxylation, which, upon further oxidation, gives calcitroic acid. However, 1,24,25(OH)D also have their own specific biological activities (Figure 2).
Figure 2: Metabolism of vitamin D.
Function in nature
Calcitriol, the active form of vitamin D, has several biological functions, some well-known and others discovered more recently. Traditionally, it regulates calcium and phosphorus levels, promoting their absorption in the gut. Vitamin D deficiency can lead to conditions like rickets in children or osteomalacia and osteoporosis in adults, while excessive supplementation can cause toxicity, resulting in hypercalcemia and kidney issues. More recently, vitamin D has been found to play an important role in both the innate and adaptive immune systems, enhancing pathogen defense and regulating T and B cell function. Additionally, calcitriol acts through the vitamin D receptor (VDR) to regulate gene expression involved in cell growth and differentiation. New insights into these mechanisms have emerged in recent years.
Derivatives and therapeutic uses
The finding that vitamin D affects cellular differentiation has led to enormous efforts in the search for new derivatives in which cellular differentiation and apoptosis activity are retained, while avoiding hypercalcemia resulting from 1,25(OH)2D itself.
It is estimated that over a thousand vitamin D derivatives have been synthesized over the years; this has led to a rapid expansion of chemical approaches. The analogues (Figure 3) that have been developed for therapeutic purposes include calcipotriol (a synthetic derivative used to treat psoriasis), paricalcitol (an analogue used to treat and prevent secondary hyperparathyroidism in patients with chronic kidney disease), doxercalciferol (another analogue used in the treatment of chronic kidney disease), tacalcitol (another synthetic derivative used to treat skin diseases), and maxacalcitol (used to treat secondary hyperparathyroidism and psoriasis).
Derivatives often contain the 1,3-dihydroxy motif on the A-ring, but additional potential modifications of that ring include the absence of the exocyclic double bond, fluorine atoms, additional hydroxyl/alkyl groups, or changes in stereochemistry. Another popular site for modification is the side chain, in which variations to the (hydroxylated) D2 and D3 motifs are numerous.
Figure 3: Synthetic analogs of vitamin D.
Synthesis in the lab
Because of the various chiral centers in vitamin D, the most efficient routes start from chiral-pool starting materials. In general, two different strategies can be used to synthesize vitamin D derivatives. The first is starting from a steroid precursor and using a biomimetic photochemical process to mimic the way nature makes these compounds (like in Figure 1). This implies that the steroid precursor already needs to have the final modifications in place, which may be challenging if different parts of the molecules need to be modified. Symeres can perform photochemistry highly efficiently under flow conditions.
For more information on our flow-chemistry capabilities, visit https://symeres.com/flow-chemistry/.
The second strategy uses the combination of two (or more) building blocks that are combined to form the triene system. A Horner–Wadsworth–Emmons reaction is often used to couple an A-ring phosphinoxide with a CD-ring ketone. For cGMP synthesis, this offers the advantage of preparing the building blocks in a multistep synthesis as regulatory starting materials, which only leaves the coupling and some deprotection and purification steps to be done under GMP conditions. As many of the vitamin D derivatives are highly active pharmaceuticals, this strategy also has the advantage of only forming the highly active species in the final steps.
Some examples of vitamin D derivatives are available in the Chiralix portfolio, see https://www.chiralix.com/en/products/vitamin-d-analogs.
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