Orally active semisynthetic

Mechanism of Action. The drug has been specifically designed as an orally active semisynthetic cephalosporin. Importantly, there are two vital reasons that are solely responsible for the oral inactivation of cephalosporins, namely ... 

Everolimus is an orally active, semisynthetic 40-O-(2-hydroxyethyl) derivative of rapamycin (also known as sirolimus) and was originally obtained from Streptomyces hygroscopicus. Everolimus is a proliferation inhibitor that blocks growth factor-mediated signal transduction and prevents organ rejection through a different mechanism than mycophenoliate mofetil (81). 

Most of the new commercial antibiotics have resulted from semisynthetic studies. New cephalosporkis, a number of which are synthesized by acylation of fermentation-derived 7-amkiocephalosporanic acid, are an example. Two orally active cephalosporkis called cefroxadine and cephalexin are produced by a synthetic ring-expansion of penicillin V. 

Today, a variety of therapeutic estrogens are produced semisynthetically from estrogen intermediates synthesized from diosgenin and other natural precursors. Two semisynthetic, orally active estrogens are ethinyl estradiol (5.27) and its 3-methyl ether (5.28, mestranol). Both of these are used in oral contraceptives (see section 5.8.3). Quinestrol (5.29) is another semisynthetic estrogen. The most important property of the semisynthetic estrogens is their increased oral effectiveness. 

It is formed by acylases that cleave off the side chain of the penicillins, and can also be obtained by the selective chemical cleavage of the amide, leaving the lactam intact. After this, 6-APA can be easily acylated by any carboxylic acid, and this has yielded literally thousands of semisynthetic penicillins in the past 30 years, many showing improved stability and activity. Some of them are lactamase resistant (methicillin (9.41), oxacillin (9.42) and its halogenated derivatives), whereas others are broad-spectrum antibiotics, like the orally active ampicillin (9.43), which also inhibits Gramnegative bacteria but is sensitive to lactamase. Carbenicillin (9.44) is particularly active against Pseudomonas and Proteus infections, which are unaffected by natural penicillins. Piperacillin (9.45), a broad-spectrum compound, is spectacularly active against Pseudomonas. 

Among the semisynthetic derivatives, cephalothin (9.47) is the most widely used since it is a broad-spectrum antibiotic resistant to lactamase. Its main drawback is that it must be injected. Cefazolin (9.48) and cephaloridine (9.49) are metabolized to a lesser extent cephalexin (9.50, analogs to ampicillin) is orally active and has a much higher acid stability than the penicillins. Cefotaxime (9.51) and moxalactam (9.52) are highly active against meningitis. 

Several hundred semisynthetic derivatives have been prepared in an effort to obtain substances with better biological activities (for references see Ref.s)). Particularly positions 3 and 4 of the naphthoquinone ring system (numbering system as proposed by Prelog7 8) have been extensively substituted, since it has been shown that structural changes in these two positions do not critically affect the action of the substance on the target enzyme, the bacterial RNA polymerase (cf. Chapter 3.). They can, however, influence other parameters such as its ability to penetrate into cells, its pharmacokinetic properties and resorption, which are all important for clinical use as an antibiotic. Rifampicin (U.S. rifampin), which is a widely used orally active tuberculostatic agent, is a 3-(4-methyl piperazinyl)-iminomethyl derivative of rifamycin SV, synthesized via the 3-formyl derivative (Fig. 5)10 ... 

SEMISYNTHETIC CEPHALOSPORINS. III. SYNTHESIS AND STRUCTURE ACTIVITY RELATIONSHIPS OF NOVEL ORALLY ACTIVE 7-[4-HYDROXY-3-(SUBSTITUTED METHYL)PHENYL]-ACETAMIDO-3-CEPHEM-4-CARBOXYLIC ACIDS... 

The majority of the semisynthetic cephalosporins are not effective when administered orally. The main exceptions are cephalexin and cephradine, and recently reported compounds such as cefaclor, cefadroxyl and cefatrizine. In view of the fact that oral activity is observed primarily in derivatives of a-amino aryl acetic acids, we designed our synthetic course on the basis of modification of the aryl part of a readily available a-amino acid, in such a way that initial functionalization of the aromatic ring would provide a "handle , which could in turn be easily converted into a variety of functional groups. The second in5)ortant criterion was the necessary optical activity of the amino acids. In order to avoid the resolution of each individual compound thus obtained, it was deemed important to start with an optically active amino acid whose modification would proceed without racemization. These two requirements were fulfilled upon chloromethylation of D-a-amino-4-hydroxyphenyl acetic acid 1. ... 

Ethinyl estradiol is a semisynthetic estrogen that has similar activity following administration by the oral and parenteral routes. 

Gindamydn has antibacterial activity similar to that of erythromycin. It exerts a bacteriostatic effect mainly on gram-positive aerobic, as well as on anaerobic pathogens. Gindamycin is a semisynthetic chloro analogue of lin-comycin, which derives from a Streptomyces species. Taken orally, clindamycin is better absorbed than lincomycin, has greater antibacterial efficacy and is thus preferred. Both penetrate well into bone tissue. 

Amoxycillin is a semisynthetic penicillin, a close congener of ampicillin and active against gram positive and negative organisms. Its absorption is more complete than ampicillin. Food does not interfere with its absorption. Its absorption after oral administration is complete hence less incidence of diarrhoea. It is eliminated in urine in unchanged form. 

Although not a taxane, ixabepilone is a novel microtubule inhibitor that was recently approved for metastatic breast cancer in combination with the oral fluoropyrimidine capecitabine or as monotherapy. It is a semisynthetic analog of epothilone B, and is active in the M phase of the cell cycle. This agent binds directly to 6-tubulin subunits on microtubules, leading to inhibition of normal microtubule dynamics. Of note, this agent continues to have activity in drug-resistant tumors that overexpress P-glycoprotein or tubulin mutations. The main adverse effects include myelosuppression, hypersensitivity reactions, and neurotoxicity in the form of peripheral sensory neuropathy. 

Hydrocodone is a semisynthetic opioid derived from codeine.18It is utilized as an analgesic and antitussive available for oral administration, often in combination with acetaminophen or ibuprofen. As a rule, potent analgesics containing a methoxyl group at position 3 (e.g., hydrocodone, K, = 19.8 nM) bind the mu receptor relatively weakly, but their O-demethylated metabolites (such as hydromorphone, Kt = 0.6 nM) bind more strangely. As with oxycodone, the possibility exists that some of their ability to relieve pain may actually derive from their active metabolites 48... 

The estrogen preparations are basically divided into three groups natural steroids, semisynthetic steroids, and nonsteroidal chemical compounds possessing estrogenic activities (Table 61.4). Trans-dermal estrogen (Estraderm) equals the efficacy of the oral preparation. 

The prototypical opioid is morphine (A.137) (Figure A.39). Isolated in a crude form, called opium, morphine has been recognized as a potent pain killer for thousands of years. Although effective, morphine has a low oral availability (F = 25%). Two common derivatives of morphine include hydrocodone (Vicodin, A.138) and oxycodone (A.139), both of which have oral availabilities of greater than 75%. Oxycodone is often sold in an oral continuous-release form under the trade name of OxyContin. Not all opioids are semisynthetic derivatives of morphine. Dextropropoxyphene (Darvon, A.140) and tramadol (Tramal, A.141) are fully synthetic opioids. Both compounds preserve the pharmacophore of morphine as described in the morphine rule (see Chapter 11). Dextropropoxyphene and tramadol are depicted in Figure A.39 to highlight possible pharmacologically active conformations that resemble morphine. 

VRLB is a semisynthetic derivative ofVLB (5 -noranhydrovinblastine), structurally distinguished from other members of its class by the modification of the cathar-anthine nucleus rather than the vindoline ring. This alteration is probably responsible for differences in its antitumor activity and tolerability profile compared with other vinca alkaloids [68]. VRLB is effective as monotherapy or in combination with a platinum derivative in patients with advanced NSCLC and advanced breast cancer [69,70]. Myelosuppression is the major dose-limiting toxicity. VRLB is administered weekly nadirs are usually reached within 14 days and patients recover within the next two weeks [71]. VRLB is also well absorbed orally. Oral and i.v. forms show similar interindividual variability, the same metabolism pattern, reproducible intrapatient blood exposure, and the same pharmacokinetic-pharmacodynamic relationship. Given at 60 mg/m /week for the first three administrations and then increased to 80 mg/m /week it achieved the same efficacy as i.v. VRLB (30 mg/m ) in terms of progression-free survival, overall survival, and objective response [70]. 

Emamectin benzoate is a semisynthetic avermectin that is derived from fermentation of avermectin B. It contains a mixture of 4" epimethylamino-4"-deoxy-avermectin B,a and Bjb benzoate salts. Emamectin benzoate is a stomach poison insecticide active against lepi-dopterous pests in vegetables, cotton, maize, peanuts, soybeans, and strawberries. Its oral LD50 in rats is 1516 mg/kg. 

Artemisia annua, known in China as Qinghaosu, contains artemisinin, which has antimalarial activity. Several derivatives of the original compound have proved effective in the treatment of Plasmodium falciparum malaria and are currently available in a variety of formulations artesunate (intravenous, rectal, oral), artelinate (oral), artemisinin (intravenous, rectal, oral), dihydroartemisinin (oral), arte-mether (intravenous, oral, rectal), and artemotil (intravenous). Artemisinic acid (qinghao acid), the precursor of artemisin, is present in the plant in a concentration up to 10 times that of artemisinin. Several semisynthetic derivatives have been developed from dihydroartemisinin (11). The artemisinin derivatives are the subject of a separate monograph. 

In the preparation of semisynthetic cephalosporins, the following improvements are sought (a) increased acid stability, (b) improved pharmacokinetic properties, particularly better oral ab.sorption, ic) broadened antimicrobial spectrum. (d) increased activity agaiast resistant microorganisms (as a result of resistance to enzymatic destruction, improved penetration. increased receptor affinity, etc.), (e) decreased allergenicity, and (f) increased tolerance after parenteral administration. 

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