旧即为新——完全合成的四环素类抗生素Eravacycline(Xerava)

“四环素抗生素在20世纪40年代开始使用。近80年来，许多细菌病原体普遍对四环素产生耐药性。幸运的是，Tetraphase Pharmaceuticals研发的一种完全合成的四环素抗生素Eravacycline（Xerava，1）于2018年获得FDA批准。与现有的其他四环素相比，它具有超强的效力，是对抗细菌感染“武器库”中的一大补充。”

1945年，美国氰化物公司的Benjamin Duggar在土壤样品中发现了第一个四环素抗生素——氯四环素（Aureomycin，2）。你看，一个人永远不会因为太老而不能做出重要的新发现。1949年，辉瑞也从泥土中发现了土霉素（Terramycin，3）。金霉素（2）和土霉素（3）是天然产物，四环素（4）也是天然产物，尽管它可以由金霉素（2）的钯催化氢化反应进行化学合成。

药物化学家在20世纪50年代开始修饰天然四环素。发现A环和B环是需要的药效团，这些极性官能团与镁离子和细菌核糖体上的极性基团形成广泛的相互作用。可喜的是，C环和D环的修饰被证明更有成效，在市场上已经生产出五种半合成的四环素抗生素。C环上C-6位置的叔醇不稳定，极易分解出非活性产物。

辉瑞（Pfizer）的多西环素（Doxylin，5，1967）和Lederle的米诺环素（Minocin，6，1972）都是为了解决不稳定性问题而发明的。二甲氨基在米诺环素（6）的D环C-7位非常有利，以至于后来几乎所有半合成和全合成的四环素都保留了这一特征。

在长达30年的时间里，惠氏公司于2005年获得FDA批准其有效的新型四环素抗生素替加环素（Tygacil，7）。另外两种半合成四环素omadacycline （Nuzyra，8）和sarrecycle（Seysara，9）由位于波士顿的Paratek Pharmaceuticals开发，并于2018年获得FDA批准。

四环素作为抗生素如何起作用?

它们通过与必需的核糖体结合来抑制细菌蛋白质的合成。

核糖体的结构如下所示。核糖体有一个夹在大亚基（50S）和小亚基（30S）蛋白质之间的mRNA分子。为了合成一种新的蛋白质，核糖体需要向正在生长的肽链中添加氨基酸，肽链才能长成蛋白质。首先，信使RNA （mRNA）编码一个新的氨基酸，然后转运RNA （tRNA）将新的氨基酸传递到生长肽链的尾部。当然，实际情况要复杂得多，但这是核糖体在蛋白质合成中的功能要点。

核糖体的结构

四环素通过与细菌核糖体的小亚基（30S）结合，选择性地抑制细菌蛋白质的合成。特别是四环素抑制氨基酰-tRNA与核糖体A位点的结合。因此，由于没有新的氨基酸可以被运送到正在生长的肽链中，蛋白质的合成被中止。人类核糖体和细菌核糖体有足够的差异，所以四环素可以选择性地对抗细菌，而不会干扰人类核糖体的功能。

左，四环素的一级结合位点（4）

右，四环素（4）和可能与16S RNA在原发位点的相互作用

四环素非常脆弱，在pH10或更高时易分解，这解释了为什么市场上有有限的半合成四环素。与现有的四环素相比，完全合成的类似物能使其与核糖体结合得更紧密。

1988年诺贝尔医学奖得主詹姆斯·布莱克（James Black）曾说过:一种新药最富有成效的基础是从一种旧药开始。这正是安德鲁·迈尔斯（Andrew Myers）教授所做的。1995年左右，加州理工学院的Myers开始了一个雄心勃勃的项目，通过全合成合成耐药性较小的四环素衍生物，从而获得不可能通过半合成获得的四环素衍生物。Myers在1998年进入哈佛大学后，这种努力仍在继续。

Myers的四环素收敛性合成创造了一个革命性的发现引擎！到目前为止，已经利用他的实用和可扩展的合成方法制备了超3000种四环素衍生物，这导致许多四环素类似物不能通过半合成的方法获得。在完全合成的四环素中，最好的可能是eravacycline（1）。从下图可以看出，eravacycline（1）在naïve和耐药菌株中明显比四环素（4）和替加环素（7）更有效。

eravacycline（1）与四环素（4）和替加环素（7）的相对效力

2013年左右，一桩“丑闻”震惊了化学界。哈佛大学研究生马克·查雷斯特（Mark Charest）起诉Myers和哈佛大学，称自己被迫接受较低版税，要求1000万美元赔偿。该诉讼后来于2016年庭外和解。

一种成功的抗生素的发现是一项伟大的科学成就，极大地造福于人类。然而，科学上的成功并不总是转化为经济上的成功。大多数专攻抗生素的生物技术公司在财务上做得都不好。例如，位于南旧金山的Achaogen Inc.在2018年获得了FDA对其新型氨基糖苷类抗生素plazomicin （Zemdri）的批准。因该药的销售非常糟糕，该公司于2019年破产。

与此同时，尽管eravacycline（Xerava, 1）的发现和开发取得了巨大的科学和医学胜利，但Tetraphase Pharmaceuticals并不是一家盈利的公司。2020年，当La Jolla Pharmaceutical Company以5900万美元收购Tetraphase时，该公司的股票价值只有几美元。

就像他们说的：善有恶报！

（向下滑动查看英文原文）

What’s Old Is New

—Totally Synthetic Tetracycline Antibiotic Eravacycline (Xerava)

Tetracycline antibiotics became available in the 1940s. Last eighty years saw widespread drug resistance against tetracyclines in many bacterial pathogens. Thankfully, a completely synthetic tetracycline antibiotic eravacycline (Xerava, 1) developed by Tetraphase Pharmaceuticals was approved by the FDA in 2018. It is hyper-potent in comparison to other existing tetracyclines and a great addition to the arsenal of weapons against bacterial infections.

The first tetracycline antibiotic chlortetracycline (Aureomycin, 2) was discovered in a soil sample by 73-year old Benjamin Duggar at American Cyanamid in 1945. You see, one is never too old to make important new discoveries. Also from dirt, Pfizer discovered oxytetracycline (Terramycin, 3) in 1949. Chlortetracycline (2) and oxytetracycline (3) are natural products, so is tetracycline (4) although it can be made chemically from palladium-catalyzed hydrogenation of chlortetracycline (2).

Medicinal chemists began to modify natural tetracyclines in the 1950s. It was found that the A and B rings are required pharmacophores—those polar functional groups form extensive interactions with the magnesium ion and polar groups on the bacterial ribosome. Gratifying, modifications of the C and D rings proved to be more fruitful, having produced five semi-synthetic tetracycline antibiotics on the market. The tertiary alcohol at the C-6 position on the C-ring is unstable, readily decomposing to give inactive products. Both doxycycline (Doxylin, 5, 1967) by Pfizer and minocycline (Minocin, 6, 1972) by Lederle were invented to address the instability issue. The dimethylamino group at the C-7 position on the D-ring of minocycline (6) is so beneficial that almost all subsequent semi-synthetic and total-synthetic tetracyclines have retained this feature. After a long inertia of 30 years, Wyeth won the FDA approval of their potent novel tetracycline antibiotic tigecycline (Tygacil, 7) in 2005. Two additional semi-synthetic tetracyclines omadacycline (Nuzyra, 8) and sarecycline (Seysara, 9) were developed by Paratek Pharmaceuticals in Boston and won the FDA approval in 2018.

How do tetracyclines work as antibiotics?

They inhibit bacterial protein synthesis by binding to the requisite ribosome.

The structure of ribosome is shown below. A ribosome has an mRNA molecule sandwiched between a large subunit (50S) and a small subunit (30S) proteins. In order to synthesize a new protein, the ribosome needs to add amino acid to the growing peptide chain, which will grow into a protein. At first, messenger RNA (mRNA) codes a new amino acid, then transfer RNA (tRNA) delivers the new amino acid to the tail of the growing peptide chain. Of course, the reality is much more complicated, but this is gist of ribosome functions in protein synthesis.

Tetracyclines selectively inhibit bacterial protein synthesis by binding to the small subunit (30S) of the bacterial ribosome. In particular, tetracyclines inhibit the binding of amino-acyl tRNA to the A site of the ribosome. As a result, protein synthesis is halted since no new amino acid can be delivered to the growing peptide chain. Human ribosomes and bacterial ribosomes have enough differences so that tetracyclines are selective against bacteria without interfering human ribosomal functions.

Tetracyclines are quite fragile. They decompose readily at pH10 or higher, which explains why limited semi-synthetic tetracyclines are available on the market. A fully synthetic analog would enable it to bind more tightly to the ribosomes than existing tetracyclines.

James Black, the 1988 Nobel Laureate in medicine, once said: The most fruitful basis of a new drug is to start with an old. That was exactly what Prof. Andrew Myers did. Around 1995, Myers at Cal Tech began an ambitious project to synthesize tetracycline derivatives with less drug resistance via total synthesis, thus gaining access to tetracyclines not possible from semi-synthesis. The efforts continued after Myers moved to Harvard in 1998. Myers’s convergent synthesis of tetracyclines created a revolutionary discovery engine! Thus far, more than 3,000 tetracycline derivatives have been prepared using his practical and scalable synthesis, which led to access of many tetracycline analogs that were not accessible via semi-synthesis. The best in the completely synthetic tetracycline is probably eravacycline (1). As you can see from the diagram below, eravacycline (1) is significantly more potent in both naïve and resistant bacterial strains than tetracycline (4) and tigecycline (7).

Around 2013, a “scandal” shocked the chemistry world. A Harvard graduate student, Mark Charest, sued Myers and Harvard for $10 million, alleging that he was coerced to accept low royalty payments. The lawsuit was later settled out of court in 2016.

Discovery of a successful antibiotic is a great scientific achievement and greatly benefits humanity. Yet, scientific success does not always translate to financial success. Most of biotech companies specializing on antibiotics have not done well financially. For instance, Achaogen Inc., a company in South San Francisco, won the FDA approval of its novel aminoglycoside antibiotic plazomicin (Zemdri) in 2018. Since sales of the drug were so abysmal that the company went bankrupt in 2019.

Meanwhile, despite a great scientific and medical triumph in discovery and development of eravacycline (Xerava, 1), Tetraphase Pharmaceuticals was not a profitable company. Its stocks were worth only a couple of dollars in 2020 when La Jolla Pharmaceutical Company acquired Tetraphase for a poultry $59 million.

Like they say: no good deed goes unpunished!