Quality of Essential Medicines from Different Sources in Enugu and Anambra, Nigeria

This study investigated the quality of 13 essential medicines in the states of Enugu and Anambra, Nigeria. A total of 260 samples were purchased from licensed pharmaceutical manufacturers and wholesalers and from vendors in pharmaceutical markets with unclear licensing status. Samples were analyzed for identity, content, and dissolution according to the United States Pharmacopeia (USP) 42 monographs. Forty-five samples of this study could be examined for authenticity with the Mobile Authentication Service scheme of the Nigerian National Agency for Food and Drug Administration and Control. Out of all samples, 25.4% did not comply with the USP 42 specifications. Strikingly, 21 out of 22 dexamethasone tablet samples (95%) were out of specification (OOS). Nine out of 19 glibenclamide samples (47%) failed dissolution testing, and 7 out of 17 cotrimoxazole samples (41%) failed assay testing. Medicines against noncommunicable diseases showed a slightly higher percentage of OOS samples than anti-infectives (21.2% versus 17.6%). The rates of OOS samples were similar in medicines stated to be produced in Nigeria, India, and China but were very different between individual manufacturers from each of these countries of origin. Therefore, prequalification of products, manufacturers, and suppliers are very important for quality assurance in medicine procurement. Unexpectedly, the total proportions of OOS samples were similar from licensed vendors (25.2%) and from markets (25.5%). Four samples (1.5%), all collected in markets, were clearly falsified and did not contain the declared active pharmaceutical ingredients. The proportion of falsified medicines was found to be lower than frequently reported in the media for Nigeria.

An open-source smartphone app for the quantitative evaluation of thin-layer chromatographic analyses in medicine quality screening

Substandard and falsified medicines present a serious threat to public health. Simple, low-cost screening tools are important in the identification of such products in low- and middle-income countries. In the present study, a smartphone-based imaging software was developed for the quantification of thin-layer chromatographic (TLC) analyses. A performance evaluation of this tool in the TLC analysis of 14 active pharmaceutical ingredients according to the procedures of the Global Pharma Health Fund (GPHF) Minilab was carried out, following international guidelines and assessing accuracy, repeatability, intermediate precision, specificity, linearity, range and robustness of the method. Relative standard deviations of 2.79% and 4.46% between individual measurements were observed in the assessments of repeatability and intermediate precision, respectively. Small deliberate variations of the conditions hardly affected the results. A locally producible wooden box was designed which ensures TLC photography under standardized conditions and shielding from ambient light. Photography and image analysis were carried out with a low-cost Android-based smartphone. The app allows to share TLC photos and quantification results using messaging apps, e-mail, cable or Bluetooth connections, or to upload them to a cloud. The app is available free of charge as General Public License (GPL) open-source software, and interested individuals or organizations are welcome to use and/or to further improve this software.

Identification of Substandard and Falsified Medicines: Influence of Different Tolerance Limits and Use of Authenticity Inquiries

Substandard and falsified medicines have severe public health and socioeconomic effects, especially in low- and middle-income countries. The WHO has emphasized the need for reliable estimates of the prevalence of such medicines to efficiently respond to this problem. In the present study, we used 601 medicine samples collected in Cameroon, the DR Congo, and Malawi to assess the rates of substandard and falsified medicines based on different criteria. Based on the specifications of the U.S. Pharmacopoeia for the amount of the active pharmaceutical ingredients, the rate of out-of-specification medicines was 9.3%. By contrast, this rate ranged from 3.3% up to 35.0% if the tolerance limits of other pharmacopoeias or recently published medicine quality studies were used. This shows an urgent need for harmonization. Principal methods to assess the rate of falsified medicines are packaging analysis, chemical analysis, and authenticity inquiries. In the present study, we carried out an authenticity inquiry for the aforementioned medicine samples, contacting 126 manufacturers and 42 distributors. Response rates were higher for samples stated to be manufactured in Asia (52.4%) or Europe (53.8%) than for samples manufactured in Africa (27.4%; P < 0.001). One sample had been identified as falsified by packaging analysis by the local researchers and two additional ones by chemical analysis. Notably, seven additional falsified samples were identified by the authenticity inquiries. The total rate of falsified medicines resulted as 1.7%. Considerations are discussed for assessing the rates of "substandard" and "falsified" medicines in future medicine quality studies.

Quality of oxytocin and misoprostol in health facilities of Rwanda

Sustainable Development Goal 3.1 calls for a reduction of the maternal mortality ratio to less than 70 per 100,000 live births by 2030. The most important cause of maternal mortality is post-partum haemorrhage (PPH). Oxytocin injections and misoprostol tablets are medicines of first choice for the management of PPH in low- and middle-income countries (LMICs). Unfortunately, both substances are chemically unstable, and previous studies have revealed serious quality problems of these medicines in LMICs. The present study is the first report on their quality in Rwanda. From 40 randomly selected health facilities (hospitals, health centers, retail pharmacies and private clinics) in different parts of Rwanda, as well as from six wholesalers and government stores, oxytocin injections and misoprostol tablets were collected. Oxytocin storage temperatures in the health facilities were monitored for six months using temperature data loggers, and found to correctly follow the storage requirements stated by the manufacturers (2–8°C, or room temperature) with few minor deviations. Oxytocin injections (57 samples, representing seven batches of four brands) were tested for their oxytocin content and pH value according to the United States Pharmacopeia. Twenty-four samples from three European manufacturers passed all tests. However, all nine samples of one batch of a Chinese manufacturer showed an excessive content of oxytocin (range 117.2–121.5% of the declared amount). Another batch of the same manufacturer showed extreme variations of the concentration of the preservative benzyl alcohol. Misoprostol tablets (25 samples, representing ten batches of six brands) were tested for content and dissolution according to the International Pharmacopoeia. Fifteen samples passed, but all 10 samples of two brands from India failed with extreme deviations, containing only 42.5–48.7% of the stated amount of misoprostol. In conclusion, oxytocin quality in Rwanda was better than reported from other African countries. However, two extremely substandard brands of misoprostol tablets were found. The Rwandan authorities reacted quickly and efficiently, and recalled these substandard medicines from the market. For oxytocin and misoprostol, with their well-known problems of quality and stability, procurement should possibly be restricted to medicines which are WHO-prequalified or which have been manufactured in countries with stringent regulatory authorities.

Quality assurance in anti-tuberculosis drug procurement by the Stop TB Partnership-Global Drug Facility: Procedures, costs, time requirements, and comparison of assay and dissolution results by manufacturers and by external analysis

BACKGROUND

Quality-assured medicines are a principal means of achieving health-related Sustainable Development Goals. An example of quality assurance/quality control (QA/QC) procedures in drug procurement is provided by the operation of the Global Drug Facility (GDF) of the Stop TB Partnership, the largest provider of tuberculosis (TB) medicines to the public sector worldwide.

METHODS

Procedures and results of GDF's quality assurance/quality control (QA/QC) over the five-year period 2013-2017 were analysed retrospectively. 13,999 batches of 51 different medicines had been procured and reviewed within this period. 1,388 of these batches had been analysed in the laboratories of GDF's external quality control agent (QCA). Assay and dissolution results determined by the manufacturers and by the external QCA were compared using Bland-Altman analysis.

RESULTS

All investigated batches of medicines were in specifications at the time of shipment. The costs for QA/QC were 0.8% of purchase costs. The median time required for chemical analysis was 10 working days. Comparison of the medicine quality analysis results showed for the poorly water-soluble compound rifampicin a bias of 4.4%, with the manufacturers reporting higher values than the external QCA, most likely due to different methods employed for the analysis. Overall 95% limits of agreement (LOAs) were -6.7 to +8.0% for assay, and -10.1 to +11.8% for dissolution. In case of kanamycin injections, 95% LOAs for assay reached -14.5 to +13.2%, largely attributable to samples from one manufacturer who had used a microbiological assay while the external QCA had used an HPLC assay.

CONCLUSIONS

GDF's procedures represent a useful benchmark when evaluating QA/QC procedures of other medicine procurement operations. Inter-laboratory comparison using Bland-Altman plots allows to investigate bias and variability in medicine quality control and should be considered as a routine procedure by drug procurement agencies, to identify priorities for further improvements.

Stability of Oxytocin Preparations in Malawi and Rwanda: Stabilizing Effect of Chlorobutanol

Oxytocin is used for the prevention and treatment of postpartum hemorrhage, the leading cause of maternal mortality in low- and middle-income countries. Because of the high instability of oxytocin, most products are labeled for storage at 2–8°C. Some other products are on the market which are labeled for non-refrigerated storage, but independent evaluations of their stability hardly exist. In the present study, seven brands (nine batches) of oxytocin were purchased from wholesalers and medical stores in Malawi and Rwanda and investigated by accelerated stability testing according to the ICH/WHO guidelines. Two oxytocin brands approved by a stringent regulatory authority (SRA) or by the WHO Prequalification of Medicines program and purchased in Europe were used as comparison. All investigated brands which were either produced in countries with SRAs, or were WHO-prequalified products, were labeled for storage at 2–8°C, and all of them passed stability testing with very good results. Even exposure to 25°C or 30°C for several months hardly affected their oxytocin content. However, two other investigated brands were labeled for non-refrigerated storage, and both of them had been produced in countries without SRAs. These two preparations showed not higher but lower stability than the brands labeled for storage at 2–8°C, and, for both of them, noncompliance with pharmacopoeial specifications was found after accelerated stability testing. At 40°C, and in forced degradation studies at 80°C, chlorobutanol showed a remarkable stabilizing effect on oxytocin, which may deserve further investigation. The results of the present study support the policy “Buy Quality Oxytocin, Keep It Cool.”

Stability of misoprostol tablets collected in Malawi and Rwanda: Importance of intact primary packaging

Misoprostol is listed in the WHO essential medicines list and can be used for induction of labour, for prevention and treatment of post-partum haemorrhage, and for abortions. The compound is unstable, and substandard misoprostol preparations have been detected in low- and middle-income countries. We now investigated the stability of misoprostol tablets according to the international guidelines for stability testing of pharmaceutical products. Three brands (four batches) of misoprostol tablets were collected in Malawi and Rwanda: the originator product, a WHO-prequalified product, and a generic product without WHO prequalification. A further batch of the originator product was collected in Germany. To investigate the effect of damage to the primary packaging, additional blister strips of one sample were intentionally damaged with a needle and investigated in parallel. Samples were placed in stability chambers for six months at 40°C/75% relative humidity (RH) and at 25°C/60% RH. After 0, 1, 2, 3 and 6 months, misoprostol content was determined according to the International Pharmacopeia. At 40°C/75% RH, all samples showed a decline of misoprostol content, but four of the batches still remained within the pharmacopeial specifications, while one of the two batches of the generic product without WHO-prequalification showed a final content of 86.2% which is out of specifications. Damage to the primary packaging greatly decreased stability, resulting in a final content of only 48.2% of the declared misoprostol amount. At 25°C/60% RH all samples remained in specifications for six months, even the sample with the damaged blister. Dissolution of misoprostol remained in specifications of the pharmacopoeia for six months for all batches, except for the sample with damaged blisters stored at 40°C/75% RH. This study confirms that the stability of misoprostol tablets must be ensured by intact, good-quality primary packaging. Careful supplier qualification is required in the procurement process.

Identification of Falsified Chloroquine Tablets in Africa at the Time of the COVID-19 Pandemic

Reports that chloroquine and hydroxychloroquine may be effective against COVID-19 have received worldwide attention, increasing the risk of the introduction of falsified versions of these medicines. Five different types of falsified chloroquine tablets were discovered between March 31, 2020 and April 4, 2020, in Cameroon and the Democratic Republic of Congo by locally conducted thin layer chromatographic analysis. Subsequent investigation by liquid chromatography and mass spectrometry in Germany proved the absence of detectable amounts of chloroquine and the presence of undeclared active pharmaceutical ingredients, that is, paracetamol and metronidazole, in four of the samples. The fifth sample contained chloroquine, but only 22% of the declared amount. Such products represent a serious risk to patients. Their occurrence exemplifies that once medicines or vaccines against COVID-19 may be developed, falsified products will enter the market immediately, especially in low- and middle-income countries (LMICs). Timely preparations for the detection of such products are required, including the establishment of appropriate screening technologies in LMICs.

Substandard and Falsified Antibiotics and Medicines against Noncommunicable Diseases in Western Cameroon and Northeastern Democratic Republic of Congo

Falsified and substandard medicines may undermine the progress toward the Sustainable Development Goals. The present study investigated the quality of 13 essential medicines in Cameroon and the Democratic Republic of Congo (DR Congo). Five hundred six medicine samples were collected from the government and faith-based health facilities, private pharmacies, and informal vendors (total 60 facilities). Collected samples were analyzed according to the U.S. Pharmacopeia (USP) for identity, content, and dissolution of their active pharmaceutical ingredients (APIs) and for uniformity of dosage units. Three samples (0.6%) were identified as falsified. Overall, 8.5% of the samples failed USP specifications for the content of the API and 11.7% failed dissolution testing. Medicines from informal vendors showed a higher out-of-specification rate (28.2%) than other types of drug outlets (12.3%; P < 0.0001). All three falsified medicines had been sold by informal vendors. The failure rate of medicines stated to be produced in Europe (5.1%) was lower than that for medicines from Asia (17.7%; P = 0.0049) and Africa (22.2%; P = 0.0042). Medicines against noncommunicable diseases showed a higher failure rate than antibiotics (25.3% versus 12.1%; P = 0.0004). Four hundred fifty-one of the samples were analyzed in Cameroon and the DR Congo with the Global Pharma Health Fund Minilab (thin-layer chromatography and disintegration testing). The three falsified medicines were readily detected in Minilab analysis. However, substandard samples were detected with low sensitivity. A well-enforced ban of medicine sales by informal vendors and increased attention to supplier qualification in the procurement process may reduce the prevalence of substandard and falsified medicines.

Quality, availability and storage conditions of oxytocin and misoprostol in Malawi

Background

Postpartum haemorrhage (PPH) is the leading cause of maternal mortality in low- and middle-income countries (LMICs). Oxytocin and misoprostol are used for the prevention and treatment of PPH. However, both medicines are chemically unstable and sensitive to environmental conditions. Previous studies reported a high prevalence of substandard oxytocin and misoprostol preparations in LMICs.

Methods

In randomly selected health facilities of four districts of Malawi, the availability of oxytocin and misoprostol was determined, and the knowledge of health workers on storage requirements and use of oxytocics was assessed. Temperature loggers were used to record the storage temperature of oxytocics. Samples of oxytocin injections and misoprostol tablets were collected from the health facilities and from wholesalers. Oxytocin samples were analysed for identity, assay (= quantity of oxytocin) and for pH value according to United States Pharmacopeia 40. Misoprostol samples were analysed for identity, assay, dissolution and related substances according to the International Pharmacopeia 2017.

Results

All visited hospitals and health centers had oxytocin available. At non-refrigerated storage sites, the recorded mean kinetic temperature exceeded the oxytocic’s storage temperature stated on the labels in 42% of the sites. At refrigerated storage sites, the required temperature of 2–8 °C was exceeded in 33% of the sites. Out of 65 oxytocin samples, 7 (11%) showed moderate deviations from specification, containing 82.2–86.8% of the declared amount of oxytocin. Out of 30 misoprostol samples, 5 (17%) showed extreme deviations, containing only 12.7–30.2% of the declared amount. The extremely substandard misoprostol was reported to the national authorities and to WHO, leading to an immediate recall of the respective brand in Malawi. The UK-based distributor of this brand closed its business shortly thereafter.

Conclusion

Availability of oxytocin was excellent in Malawi, and its quality was better than reported in previous studies in other LMICs. However, storage conditions at the health facilities often did not meet the requirements. Extremely substandard misoprostol tablets were found, representing a serious risk to maternal health. This shows the need for continued efforts for quality assurance in medicine procurement and registration, as well as for post-marketing surveillance.

Availability, prices and affordability of selected antibiotics and medicines against non-communicable diseases in western Cameroon and northeast DR Congo

Access to safe, effective and affordable medicines of good quality is included into the Sustainable Development Goals of the United Nations. Furthermore, WHO has developed a Global Action Plan with the aim to raise access to essential medicines against non-communicable diseases (NCDs) to 80%, and to improve their affordability. In order to contribute to the monitoring of progress towards these goals, the present study investigated the availability and affordability of seven antibiotics and six medicines against non-communicable diseases in the northeast of the Democratic Republic of Congo and the west of the Republic of Cameroon. Data on availability and prices of these medicines were collected in 60 different sites (34 in the DR Congo, 26 in Cameroon), including government health facilities, church health facilities, private pharmacies and informal vendors, as part of a study on medicine quality. The data were analyzed using a standardized procedure developed by WHO and Health Action International (HAI). Average availability of the investigated antibiotics ranged from 62% to 98% in the different types of facilities in both countries, including the informal vendors. Average availability for medicines against NCDs in the different types of facilities showed a higher variation in both countries, ranging from 11% up to 87%. The average availability of medicines against NCDs in government health facilities was only 33% in Cameroon, and as low as 11% in the DR Congo. In contrast, availability of medicines against NCDs in church health facilities in Cameroon was 70%, not far from the 80% availability goal set by WHO. Medicine prices were clearly higher in Cameroon than in the DR Congo, with median price ratios to an international reference price of 5.69 and 2.17, respectively (p < 0.001). In relation to the daily minimum wages in both countries, treatment courses with five of the seven investigated antibiotics could be considered as affordable, while in each country only one out of the five investigated medicines against NCDs could be considered as affordable. Especially generic medicines provided by government and church health facilities showed reasonable affordability in most cases, while originator medicines offered by private pharmacies were clearly unaffordable to a major part of the population. Despite some encouraging findings on the availability of antibiotics in both countries, the availability and affordability of medicines against NCDs urgently requires further improvements.

Quality of medicines in southern Togo: Investigation of antibiotics and of medicines for non-communicable diseases from pharmacies and informal vendors

Substandard and falsified medicines represent a serious threat for public health and patient safety. Especially in low and middle-income countries, the prevalence of substandard and falsified medicines is reportedly high. However, reliable information on the prevalence of poor-quality medicines is scarce. In this study, 12 essential medicines, including antibiotics, antidiabetics, cardiac drugs and antiasthmatic drugs, were collected from six informal vendors and six licensed pharmacies in the southern part of Togo (regions Maritime and Plateaux). A mystery shopper approach was used in both types of outlets. In total, 64 samples were collected from licensed pharmacies and 30 from informal vendors. Both availability of medicines and prices of medicines were higher in licensed pharmacies than in informal vendors. 92 medicine samples were analyzed by visual examination, followed by chemical analysis for the content and for the dissolution of the active pharmaceutical ingredients according to the respective monographs of the United States Pharmacopoeia. 7 samples (8%) did not comply with the pharmacopoeial specifications, and one sample (1%) showed even extreme deviations. None of the samples was obviously falsified. However, one sample of amoxicillin capsules contained only 47% of the declared content of the active pharmaceutical ingredient, indicating that it may represent amoxicillin capsules 250 mg, rather than 500mg as declared on the label. Medicines stated to originate from Asia (i.e. mainly from India and China) showed a significantly higher proportion (24%) of non-compliant samples than those from Africa and Europe (4%, p = 0.007). High failure rates were observed in medicines both from informal vendors (13%) and from licensed pharmacies (5%), but the difference between both groups was not statistically significant (p = 0.152). The observed high prevalence of substandard medicines requires action from regulatory authorities and health care providers. Testing of selected samples for related substances indicated that inappropriate transport and storage conditions may have been an important cause for substandard quality.

Surveillance for falsified and substandard medicines in Africa and Asia by local organizations using the low-cost GPHF Minilab

Background

Substandard and falsified medical products present a serious threat to public health, especially in low- and middle-income countries. Their identification using pharmacopeial analysis is expensive and requires sophisticated equipment and highly trained personnel. Simple, low-cost technologies are required in addition to full pharmacopeial analysis in order to accomplish widespread routine surveillance for poor-quality medicines in low- and middle-income countries.

Methods

Ten faith-based drug supply organizations in seven countries of Africa and Asia were each equipped with a Minilab of the Global Pharma Health Fund (GPHF, Frankfurt, Germany), suitable for the analysis of about 85 different essential medicines by thin-layer chromatography. Each organization was asked to collect approximately 100 medicine samples from private local medicine outlets, especially from the informal sector. The medicine samples were tested locally according to the Minilab protocols. Medicines which failed Minilab testing were subjected to confirmatory analysis in a WHO-prequalified medicine quality control laboratory in Kenya.

Results

Out of 869 medicine samples, 21 were confirmed to be substandard or falsified medical products. Twelve did not contain the stated active pharmaceutical ingredient (API), six contained insufficient amounts of the API, and three showed insufficient dissolution of the API. The highest proportion of substandard and falsified medicines was found in Cameroon (7.1%), followed by the Democratic Republic of Congo (2.7%) and Nigeria (1.1%). Antimalarial medicines were most frequently found to be substandard or falsified (9.5% of all antimalarials). Thin-layer chromatography according to the Minilab protocols was found to be specific and reproducible in the identification of medicines which did not contain the stated API. Since only samples which failed Minilab testing were subjected to confirmatory testing using pharmacopeial methods, this study did not assess the sensitivity of the Minilab methodology in the detection of substandard medicines, and may underestimate the prevalence of poor-quality medicines.

Conclusions

Surveillance for poor-quality medicines can be carried out by local organizations in low- and middle-income countries using a simple, low-cost technology. Such surveillance can identify an important subgroup of the circulating substandard and falsified medical products and can help to prevent them from causing harm in patients. A collaboration of the national drug regulatory authorities with faith-based organizations and other NGOs may therefore represent a promising strategy towards the Sustainable Development Goal of “ensuring access to quality medicines”.

Low Prevalence of Substandard and Falsified Antimalarial and Antibiotic Medicines in Public and Faith-Based Health Facilities of Southern Malawi

AbstractSubstandard and falsified antimalarial and antibiotic medicines represent a serious problem for public health, especially in low- and middle-income countries. However, information on the prevalence of poor-quality medicines is limited. In the present study, samples of six antimalarial and six antibiotic medicines were collected from 31 health facilities and drug outlets in southern Malawi. Random sampling was used in the selection of health facilities. For sample collection, an overt approach was used in licensed facilities, and a mystery shopper approach in nonlicensed outlets. One hundred and fifty-five samples were analyzed by visual and physical examination and by rapid prescreening tests, that is, disintegration testing and thin-layer chromatography using the GPHF-Minilab. Fifty-six of the samples were analyzed according to pharmacopeial monographs in a World Health Organization-prequalified quality control laboratory. Seven out-of-specification medicines were identified. One sample was classified as falsified, lacking the declared active ingredients, and containing other active ingredients instead. Three samples were classified as substandard with extreme deviations from the pharmacopeial standards, and three further samples as substandard with nonextreme deviations. Of the substandard medicines, three failed in dissolution testing, two in the assay for the content of the active pharmaceutical ingredient, and one failed in both dissolution testing and assay. Six of the seven out-of-specification medicines were from private facilities. Only one out-of-specification medicine was found within the samples from public and faith-based health facilities. Although the observed presence of substandard and falsified medicines in Malawi requires action, their low prevalence in public and faith-based health facilities is encouraging.

Availability and affordability of antimalarial and antibiotic medicines in Malawi

Background

Availability and affordability of medicines are key determinants of universal health coverage, yet achieving them presents a major challenge especially in low-income countries. We here present an analysis of availability and prices of antimalarial and antibiotic medicines in public, faith-based and private health facilities in Malawi. Medicines are provided free of charge in the public health care system of Malawi. In contrast, facilities of the Christian Health Association of Malawi (CHAM) usually charge their patients for medicines, as do private for-profit facilities.

Methods

As part of a study on medicine quality, samples of six antimalarial and six antibiotic medicines were collected in 31 health facilities in four districts of southern Malawi. These included 15 public facilities (i.e. health centres, district hospitals and central hospitals), eight CHAM and eight private facilities. Random selection was used in choosing the included health facilities. The availability of medicines was recorded, including the number of units which could be collected of each medicine, as well as the prices of medicines which were charged in CHAM and private facilities. These data were analyzed using the standard methodology developed by the World Health Organization (WHO) and Health Action International (HAI).

Results

Availability of the antimalarials artemether/lumefantrine and sulfadoxine/pyrimethamine, which are provided with financial support from international donors, was high in public and CHAM facilities (93% and 100%, respectively). However, availability of antibiotics was much lower (e.g. 40% availability of amoxicillin tablets/capsules in public health centres). Medicine prices were lower than reported from many other countries. The median price ratio (MPR) to a wholesale international procurement price was 2.8 in CHAM facilities and even lower in the private sector (MPR 2.3). Nevertheless, for 10 of the 12 investigated medicines the cost for one course of treatment exceeded the daily wage of a low-paid government worker in Malawi and therefore had to be considered as unaffordable for a major part of the population.

Conclusions

Continued efforts are required to improve the availability of essential medicines in Malawi. The free provision of medicines in the public health care system remains important in order to achieve universal health coverage, due to the low income in this country.

Use of thin-layer chromatography to detect counterfeit sulfadoxine/pyrimethamine tablets with the wrong active ingredient in Malawi

Background

Substandard and falsified anti-malarial medicines pose a serious threat to public health, especially in low-income countries. Appropriate technologies for drug quality analysis in resource-limited settings are important for the surveillance of the formal and informal drug market. The feasibility of thin-layer chromatography (TLC) with different solvent systems was tested using the GPHF Minilab in a study of the quality of sulfadoxine/pyrimethamine tablets in Malawi.

Methods

Twenty eight samples of sulfadoxine/pyrimethamine tablets were collected from randomly selected health facilities of four districts of southern Malawi. A mystery shopper approach was used when collecting samples from illegal street vendors, and an overt approach for the other facilities. Samples were subjected to visual inspection, disintegration testing and TLC analysis. 10 samples were further investigated according to the methods of the US Pharmacopeia using high performance liquid chromatography (HPLC).

Results

One sample was found to be falsified, containing a mixture of paracetamol tablets and co-trimoxazole tablets. These had been repackaged into paper strip packs labelled as a brand of sulfadoxine/pyrimethamine. TLC with different solvent systems readily proved that these tablets did not comply with their declaration, and provided strong evidence for the active pharmaceutical ingredients which were actually contained. Full pharmacopeial analysis by HPLC confirmed the results suggested by TLC for this sample, and showed two further samples to be of substandard quality.

Conclusions

Due to the absence of the declared anti-malarial ingredients and due to the presence of other pharmaceutical ingredients, the identified falsified medicine represents a serious health risk for the population. Thin-layer chromatography (TLC) using different solvent systems proved to be a powerful method for the identification of this type of counterfeiting, presenting a simple and affordable technology for use in resource-limited settings.

Diversity of ABBA Prenyltransferases in Marine Streptomyces sp. CNQ-509: Promiscuous Enzymes for the Biosynthesis of Mixed Terpenoid Compounds

Terpenoids are arguably the largest and most diverse family of natural products, featuring prominently in e.g. signalling, self-defence, UV-protection and electron transfer. Prenyltransferases are essential players in terpenoid and hybrid isoprenoid biosynthesis that install isoprene units on target molecules and thereby often modulate their bioactivity. In our search for new prenyltransferase biocatalysts we focused on the marine-derived Streptomyces sp. CNQ-509, a particularly rich source of meroterpenoid chemistry. Sequencing and analysis of the genome of Streptomyces sp. CNQ-509 revealed seven putative phenol/phenazine-specific ABBA prenyltransferases, and one putative indole-specific ABBA prenyltransferase. To elucidate the substrate specificity of the ABBA prenyltransferases and to learn about their role in secondary metabolism, CnqP1 -CnqP8 were produced in Escherichia coli and incubated with various aromatic and isoprenoid substrates. Five of the eight prenyltransferases displayed enzymatic activity. The efficient conversion of dihydroxynaphthalene derivatives by CnqP3 (encoded by AA958_24325) and the co-location of AA958_24325 with genes characteristic for the biosynthesis of THN (tetrahydroxynaphthalene)-derived natural products indicates that the enzyme is involved in the formation of debromomarinone or other naphthoquinone-derived meroterpenoids. Moreover, CnqP3 showed high flexibility towards a range of aromatic and isoprenoid substrates and thus represents an interesting new tool for biocatalytic applications.

Genome-based discovery of a novel membrane-bound 1,6-dihydroxyphenazine prenyltransferase from a marine actinomycete

Recently, novel prenylated derivatives of 1,6-dihydroxyphenazine have been isolated from the marine sponge-associated Streptomyces sp. SpC080624SC-11. Genome sequencing of this strain now revealed a gene cluster containing all genes necessary for the synthesis of the phenazine and the isoprenoid moieties. Unexpectedly, however, the cluster did not contain a gene with similarity to previously investigated phenazine prenyltransferases, but instead a gene with modest similarity to the membrane-bound prenyltransferases of ubiquinone and menaquinone biosynthesis. Expression of this gene in E. coli and isolation of the membrane fraction proved that the encoded enzyme, Mpz10, catalyzes two successive prenylations of 1,6-dihydroxyphenazine. Mpz10 is the first example of a membrane-bound enzyme catalyzing the prenylation of a phenazine substrate, and one of few examples of membrane-bound enzymes involved in the prenylation of aromatic secondary metabolites in microorganisms.

Versuche zur Synthese natürlich vorkommender prenylierter Naphthalinderivate. Nachweis eines neuen Prenylnaphthochinon- derivates in Galium mollugo / Synthesis of Naturally Occurring Prenylated Naphthalene Derivates. Isolation of a New Prenylnaphthoquinone from Galium mollugo

Several prenylated naphthalene derivatives were synthesized and characterized by spectroscopic methods. One of these compounds, 2-methoxycarbonyl-3-prenyl-1,4-naphthoquinone, was detected as a new natural product in the root system of Galium mollugo L. This new natural product is likely to be related to the biosynthesis of mollugin and anthraquinones.

New aminocoumarins from the rare actinomycete Catenulispora acidiphila DSM 44928: identification, structure elucidation, and heterologous production

Genome mining led to the discovery of a novel aminocoumarin gene cluster in the rare actinomycete Catenulispora acidiphila DSM 44928. Sequence analysis revealed the presence of genes putatively involved in export/resistance, regulation, and biosynthesis of the aminocoumarin moiety and its halogenation, as well as several genes with so far unknown function. Two new aminocoumarins, cacibiocin A and B, were identified in the culture broth of C. acidiphila. Heterologous expression of the putative gene cluster in Streptomyces coelicolor M1152 confirmed that this cluster is responsible for cacibiocin biosynthesis. Furthermore, total production levels of cacibiocins could be increased by heterologous expression and screening of different culture media from an initial yield of 4.9 mg L(-1) in C. acidiphila to 60 mg L(-1) in S. coelicolor M1152. By HR-MS and NMR analysis, cacibiocin A was found to contain a 3-amino-4,7-dihydroxycoumarin moiety linked by an amide bond to a pyrrole-2,5-dicarboxylic acid. The latter structural motif has not been identified previously in any natural compound. Additionally, cacibiocin B contains two chlorine atoms at positions 6' and 8' of the aminocoumarin moiety.

Unusual N-prenylation in diazepinomicin biosynthesis: the farnesylation of a benzodiazepine substrate is catalyzed by a new member of the ABBA prenyltransferase superfamily

The bacterium Micromonospora sp. RV115, isolated from a marine sponge, produces the unusual metabolite diazepinomicin, a prenylated benzodiazepine derivative. We have cloned the prenyltransferase gene dzmP from this organism, expressed it in Escherichia coli, and the resulting His₈-tagged protein was purified and investigated biochemically. It was found to catalyze the farnesylation of the amide nitrogen of dibenzodiazepinone. DzmP belongs to the ABBA prenyltransferases and is the first member of this superfamily which utilizes farnesyl diphosphate as genuine substrate. All previously discovered members utilize either dimethylallyl diphosphate (C₅) or geranyl diphosphate (C₁₀). Another putative diazepinomicin biosynthetic gene cluster was identified in the genome of Streptomyces griseoflavus Tü4000, suggesting that the formation of diazepinomicin is not restricted to the genus Micromonospora. The gene cluster contains a gene ssrg_00986 with 61.4% identity (amino acid level) to dzmP. The gene was expressed in E. coli, and the purified protein showed similar catalytic properties as DzmP. Both enzymes also accepted other phenolic or phenazine substrates. ABBA prenyltransferases are useful tools for chemoenzymatic synthesis, due to their nature as soluble, stable biocatalysts. The discovery of DzmP and Ssrg_00986 extends the isoprenoid substrate range of this superfamily. The observed prenylation of an amide nitrogen is an unusual biochemical reaction.

Five gene products are required for assembly of the central pyrrole moiety of coumermycin A1

Coumermycin A1 is an aminocoumarin antibiotic produced by Streptomyces rishiriensis. It exhibits potent antibacterial and anticancer activity. The coumermycin A1 molecule contains two terminal 5-methyl-pyrrole-2-carboxylic acid moieties and one central 3-methylpyrrole-2,4-dicarboxylic acid moiety (CPM). While the biosynthesis of the terminal moieties has been elucidated in detail, the pathway leading to the CPM remains poorly understood. In this work, the minimal set of genes required for the generation of the CPM scaffold was identified. It comprises the five genes couR1, couR2a, couR2b, couR3, and couR4 which are grouped together in a contiguous 4.7 kb region within the coumermycin A1 biosynthetic gene cluster. The DNA fragment containing these genes was cloned into an expression plasmid and heterologously expressed in Streptomyces coelicolor M1146. Thereupon, the formation of CPM could be shown by HPLC and by HPLC-MS/MS, in comparison to an authentic CPM standard. This proves that the genes couR1–couR4 are sufficient to direct the biosynthesis of CPM, and that the adjacent genes couR5 and couR6 are not required for this pathway. The enzyme CouR3 was expressed in Escherichia coli and purified to near homogeneity. The protein exhibited an ATPase activity similar to that reported for its close ortholog, the threonine kinase PduX. However, we could not show a threonine kinase activity of CouR3, and; therefore, the substrate of CouR3 in CPM biosynthesis is still unknown and may be different from threonine.

Phage p1-derived artificial chromosomes facilitate heterologous expression of the FK506 gene cluster

We describe a procedure for the conjugative transfer of phage P1-derived Artificial Chromosome (PAC) library clones containing large natural product gene clusters (≥70 kilobases) to Streptomyces coelicolor strains that have been engineered for improved heterologous production of natural products. This approach is demonstrated using the gene cluster for FK506 (tacrolimus), a clinically important immunosuppressant of high commercial value. The entire 83.5 kb FK506 gene cluster from Streptomyces tsukubaensis NRRL 18488 present in one 130 kb PAC clone was introduced into four different S. coelicolor derivatives and all produced FK506 and smaller amounts of the related compound FK520. FK506 yields were increased by approximately five-fold (from 1.2 mg L^-1 to 5.5 mg L^-1) in S. coelicolor M1146 containing the FK506 PAC upon over-expression of the FK506 LuxR regulatory gene fkbN. The PAC-based gene cluster conjugation methodology described here provides a tractable means to evaluate and manipulate FK506 biosynthesis and is readily applicable to other large gene clusters encoding natural products of interest to medicine, agriculture and biotechnology.

Structural basis of the interaction of MbtH-like proteins, putative regulators of nonribosomal peptide biosynthesis, with adenylating enzymes

The biosynthesis of nonribosomally formed peptides (NRPs), which include important antibiotics such as vancomycin, requires the activation of amino acids through adenylate formation. The biosynthetic gene clusters of NRPs frequently contain genes for small, so-called MbtH-like proteins. Recently, it was discovered that these MbtH-like proteins are required for some of the adenylation reactions in NRP biosynthesis, but the mechanism of their interaction with the adenylating enzymes has remained unknown. In this study, we determined the structure of SlgN1, a 3-methylaspartate-adenylating enzyme involved in the biosynthesis of the hybrid polyketide/NRP antibiotic streptolydigin. SlgN1 contains an MbtH-like domain at its N terminus, and our analysis defines the parameters required for an interaction between MbtH-like domains and an adenylating enzyme. Highly conserved tryptophan residues of the MbtH-like domain critically contribute to this interaction. Trp-25 and Trp-35 form a cleft on the surface of the MbtH-like domain, which accommodates the alanine side chain of Ala-433 of the adenylating domain. Mutation of Ala-433 to glutamate abolished the activity of SlgN1. Mutation of Ser-23 of the MbtH-like domain to tyrosine resulted in strongly reduced activity. However, the activity of this S23Y mutant could be completely restored by addition of the intact MbtH-like protein CloY from another organism. This suggests that the interface found in the structure of SlgN1 is the genuine interface between MbtH-like proteins and adenylating enzymes.

Structure-based engineering increased the catalytic turnover rate of a novel phenazine prenyltransferase

Prenyltransferases (PTs) catalyze the regioselective transfer of prenyl moieties onto aromatic substrates in biosynthetic pathways of microbial secondary metabolites. Therefore, these enzymes contribute to the chemical diversity of natural products. Prenylation is frequently essential for the pharmacological properties of these metabolites, including their antibiotic and antitumor activities. Recently, the first phenazine PTs, termed EpzP and PpzP, were isolated and biochemically characterized. The two enzymes play a central role in the biosynthesis of endophenazines by catalyzing the regiospecific prenylation of 5,10-dihydrophenazine-1-carboxylic acid (dhPCA) in the secondary metabolism of two different Streptomyces strains. Here we report crystal structures of EpzP in its unliganded state as well as bound to S-thiolodiphosphate (SPP), thus defining the first three-dimensional structures for any phenazine PT. A model of a ternary complex resulted from in silico modeling of dhPCA and site-directed mutagenesis. The structural analysis provides detailed insight into the likely mechanism of phenazine prenylation. The catalytic mechanism suggested by the structure identifies amino acids that are required for catalysis. Inspection of the structures and the model of the ternary complex furthermore allowed us to rationally engineer EpzP variants with up to 14-fold higher catalytic reaction rate compared to the wild-type enzyme. This study therefore provides a solid foundation for additional enzyme modifications that should result in efficient, tailor-made biocatalysts for phenazines production.

Mutational analysis of a phenazine biosynthetic gene cluster in Streptomyces anulatus 9663

The biosynthetic gene cluster for endophenazines, i.e., prenylated phenazines from Streptomyces anulatus 9663, was heterologously expressed in several engineered host strains derived from Streptomyces coelicolor M145. The highest production levels were obtained in strain M512. Mutations in the rpoB and rpsL genes of the host, which result in increased production of other secondary metabolites, had no beneficial effect on the production of phenazines. The heterologous expression strains produced, besides the known phenazine compounds, a new prenylated phenazine, termed endophenazine E. The structure of endophenazine E was determined by high-resolution mass spectrometry and by one- and two-dimensional NMR spectroscopy. It represented a conjugate of endophenazine A (9-dimethylallylphenazine-1-carboxylic acid) and L-glutamine (L-Gln), with the carboxyl group of endophenazine A forming an amide bond to the α-amino group of L-Gln. Gene inactivation experiments in the gene cluster proved that ppzM codes for a phenazine N-methyltransferase. The gene ppzV apparently represents a new type of TetR-family regulator, specifically controlling the prenylation in endophenazine biosynthesis. The gene ppzY codes for a LysR-type regulator and most likely controls the biosynthesis of the phenazine core. A further putative transcriptional regulator is located in the vicinity of the cluster, but was found not to be required for phenazine or endophenazine formation. This is the first investigation of the regulatory genes of phenazine biosynthesis in Streptomyces.

Evolutionary relationships of microbial aromatic prenyltransferases

The linkage of isoprenoid and aromatic moieties, catalyzed by aromatic prenyltransferases (PTases), leads to an impressive diversity of primary and secondary metabolites, including important pharmaceuticals and toxins. A few years ago, a hydroxynaphthalene PTase, NphB, featuring a novel ten-stranded β-barrel fold was identified in Streptomyces sp. strain CL190. This fold, termed the PT-barrel, is formed of five tandem ααββ structural repeats and remained exclusive to the NphB family until its recent discovery in the DMATS family of indole PTases. Members of these two families exist only in fungi and bacteria, and all of them appear to catalyze the prenylation of aromatic substrates involved in secondary metabolism. Sequence comparisons using PSI-BLAST do not yield matches between these two families, suggesting that they may have converged upon the same fold independently. However, we now provide evidence for a common ancestry for the NphB and DMATS families of PTases. We also identify sequence repeats that coincide with the structural repeats in proteins belonging to these two families. Therefore we propose that the PT-barrel arose by amplification of an ancestral ααββ module. In view of their homology and their similarities in structure and function, we propose to group the NphB and DMATS families together into a single superfamily, the PT-barrel superfamily.

Role of MbtH-like proteins in the adenylation of tyrosine during aminocoumarin and vancomycin biosynthesis

MbtH-like proteins consist of ∼70 amino acids and are encoded in the biosynthetic gene clusters of non-ribosomally formed peptides and other secondary metabolites derived from amino acids. Recently, several MbtH-like proteins have been shown to be required for the adenylation of amino acid in non-ribosomal peptide synthesis. We now investigated the role of MbtH-like proteins in the biosynthesis of the aminocoumarin antibiotics novobiocin, clorobiocin, and simocyclinone D8 and of the glycopeptide antibiotic vancomycin. The tyrosine-adenylating enzymes CloH, SimH, and Pcza361.18, involved in the biosynthesis of clorobiocin, simocyclinone D8, and vancomycin, respectively, required the presence of MbtH-like proteins in a 1:1 molar ratio, forming heterotetrameric complexes. In contrast, NovH, involved in novobiocin biosynthesis, showed activity in the absence of MbtH-like proteins. Comparison of the active centers of CloH and NovH showed only one amino acid to be different, i.e. Leu-383 versus Met-383. Mutation of this amino acid in CloH (L383M) indeed led to MbtH-independent adenylating activity. All investigated tyrosine-adenylating enzymes exhibited remarkable promiscuity for MbtH-like proteins from different pathways and organisms. YbdZ, the MbtH-like protein from the expression host Escherichia coli, was found to bind to adenylating enzymes during expression and to influence their biochemical properties markedly. Therefore, the use of ybdZ-deficient expression hosts is important in biochemical studies of adenylating enzymes.

The biosynthetic genes for prenylated phenazines are located at two different chromosomal loci of Streptomyces cinnamonensis DSM 1042

Streptomyces cinnamonensis DSM 1042 produces two types of isoprenoid secondary metabolites: the prenylated naphthalene derivative furanonaphthoquinone I (FNQ I), and isoprenylated phenazines which are termed endophenazines. Previously, a 55 kb gene cluster was identified which contained genes for both FNQ I and endophenazine biosynthesis. However, several genes required for the biosynthesis of these metabolites were not present in this cluster. We now re-screened the cosmid library for genes of the mevalonate pathway and identified a separate genomic locus which contains the previously missing genes. This locus (15 kb) comprised orthologues of four phenazine biosynthesis genes known from Pseudomonas strains. Furthermore, the locus contained a putative operon of six genes of the mevalonate pathway, as well as the gene epzP which showed sequence similarity to a recently discovered class of prenyltransferases. Inactivation and complementation experiments proved the involvement of epzP in the prenylation reaction in endophenazine biosynthesis. This newly identified genomic locus is more than 40 kb distant from the previously identified cluster. The protein EpzP was expressed in Escherichia coli in form of a his-tag fusion protein and purified. The enzyme catalysed the prenylation of 5,10-dihydrophenazine-1-carboxylic acid (dihydro-PCA) using dimethylallyl diphosphate (DMAPP) as isoprenoid substrate. K(m) values were determined as 108 µM for dihydro-PCA and 25 µM for DMAPP.

Identification and structural elucidation of new caprazamycins from Streptomyces sp. MK730-62F2 by liquid chromatography/electrospray ionization tandem mass spectrometry

The development of reliable analytic methods, capable of separating mixtures of secondary metabolites as well as providing structural information, is essential for the investigation of secondary metabolites, e.g. from Streptomyces. Here we report a liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method using a triple quadrupole mass analyzer for the structural elucidation of caprazamycins and liposidomycins from culture extracts of the wild-type producer strains. Comparison of the fragmentation patterns in positive as well as in negative ionization mode revealed several characteristic product ions used for identification of six new caprazamycins. Furthermore, a chromatographic method for the purification of nucleosides from cell cultures using a boronic acid gel was adapted for the partial purification of the culture extracts.

Structure and mechanism of the magnesium-independent aromatic prenyltransferase CloQ from the clorobiocin biosynthetic pathway

CloQ is an aromatic prenyltransferase from the clorobiocin biosynthetic pathway of Streptomyces roseochromogenes var. oscitans. It is involved in the synthesis of the prenylated hydroxybenzoate moiety of the antibiotic, specifically catalyzing the attachment of a dimethylallyl moiety to 4-hydroxyphenylpyruvate. Herein, we report the crystal structure of CloQ and use it as a framework for interpreting biochemical data from both wild-type and variant proteins. CloQ belongs to the aromatic prenyltransferase family, which is characterized by an unusual core fold comprising five consecutive ααββ elements that form a central 10-stranded anti-parallel β-barrel. The latter delineates a solvent-accessible cavity where substrates bind and catalysis takes place. This cavity has well-defined polar and nonpolar regions, which have distinct roles in substrate binding and facilitate a Friedel-Crafts-type mechanism. We propose that the juxtaposition of five positively charged residues in the polar region circumvents the necessity for a Mg(2+), which, by contrast, is a strict requirement for the majority of prenyltransferases characterized to date. Our structure of CloQ complexed with 4-hydroxyphenylpyruvate reveals the formation of a covalent link between the substrate and Cys215 to yield a thiohemiketal species. Through site-directed mutagenesis, we show that this link is not essential for enzyme activity in vitro. Furthermore, we demonstrate that CloQ will accept alternative substrates and, therefore, has the capacity to generate a range of prenylated compounds. Since prenylation is thought to enhance the bioactivity of many natural products, CloQ offers considerable promise as a biocatalyst for the chemoenzymatic synthesis of novel compounds with therapeutic potential.

A new group of aromatic prenyltransferases in fungi, catalyzing a 2,7-dihydroxynaphthalene 3-dimethylallyl-transferase reaction

Five fungal genomes from the Ascomycota (sac fungi) were found to contain a gene with sequence similarity to a recently discovered small group of bacterial prenyltransferases that catalyze the C-prenylation of aromatic substrates in secondary metabolism. The genes from Aspergillus terreus NIH2624, Botryotinia fuckeliana B05.10 and Sclerotinia sclerotiorum 1980 were expressed in Escherichia coli, and the resulting His(8)-tagged proteins were purified and investigated biochemically. Their substrate specificity was found to be different from that of any other prenyltransferase investigated previously. Using 2,7-dihydroxynaphthalene (2,7-DHN) and dimethylallyl diphosphate as substrates, they catalyzed a regiospecific Friedel-Crafts alkylation of 2,7-DHN at position 3. Using the enzyme of A. terreus, the K(m) values for 2,7-DHN and dimethylallyl diphosphate were determined as 324 +/- 25 microM and 325 +/- 35 microM, respectively, and k(cat) as 0.026 +/- 0.001 s(-1). A significantly lower level of prenylation activity was found using dihydrophenazine-1-carboxylic acid as aromatic substrate, and only traces of products were detected with aspulvinone E, flaviolin, or 4-hydroxybenzoic acid. No product was formed with l-tryptophan, l-tyrosine, or 4-hydroxyphenylpyruvate. The genes for these fungal prenyltransferases are not located within recognizable secondary metabolic gene clusters. Their physiological function is yet unknown.

Prenyl transfer to aromatic substrates in the biosynthesis of aminocoumarins, meroterpenoids and phenazines: the ABBA prenyltransferase family

Aromatic prenyltransferases transfer prenyl moieties onto aromatic acceptor molecules, catalyzing an electrophilic substitution of the aromatic ring under formation of carbon-carbon bonds. They give rise to an astounding diversity of primary and secondary metabolites in plants, fungi and bacteria. This review describes a recently discovered family of aromatic prenyltransferases. The structure of these enyzmes shows a type of beta/alpha fold with antiparallel beta strands. Due to the alpha-beta-beta-alpha architecture of this fold, this group of enzymes was designated as ABBA prenyltransferases. They lack the (N/D)DxxD motif which is characteristic for many other prenyltransferases. At present, 14 genes with sequence similarity to ABBA prenyltransferases can be identified in the database. A phylogenetic analysis of these genes separates them into two clades. One of them comprises the 4-hydroxyphenylpyruvate 3-dimethylallyltransferases CloQ and NovQ involved in aminocoumarin antibiotic biosynthesis in Streptomyces strains, as well as four genes of unknown function from fungal genomes. The other clade comprises genes involved in the biosynthesis of prenylated naphthoquinones and prenylated phenazines in different streptomycetes. ABBA prenyltransferases are soluble biocatalysts which can easily be obtained as homogeneous proteins in significant amounts. Their substrates are accommodated in a surprisingly spacious central cavity which explains their promiscuity for different aromatic substrates. Therefore, the enzymes of this family represent attractive tools for the chemoenzymatic synthesis of bioactive molecules.

Transcriptional regulation of the novobiocin biosynthetic gene cluster

The aminocoumarin antibiotic novobiocin is a gyrase inhibitor formed by a Streptomyces strain. The biosynthetic gene cluster of novobiocin spans 23.4 kb and contains 20 coding sequences, among them the two regulatory genes novE and novG. We investigated the location of transcriptional promoters within this cluster by insertion of transcriptional terminator cassettes and RT-PCR analysis of the resulting mutants. The cluster was found to contain eight DNA regions with promoter activity. The regulatory protein NovG binds to a previously identified binding site within the promoter region located upstream of novH, but apparently not to any of the other seven promoters. Quantitative real-time PCR was used to compare the number of transcripts in a strain carrying an intact novobiocin cluster with strains carrying mutated clusters. Both in-frame deletion of the regulatory gene novG and insertion of a terminator cassette into the biosynthetic gene novH led to a strong reduction of the number of transcripts of the genes located between novH and novW. This suggested that these 16 biosynthetic genes form a single operon. Three internal promoters are located within this operon but appear to be of minor importance, if any, under our experimental conditions. Transcription of novG was found to depend on the presence of NovE, suggesting that the two regulatory genes, novE and novG, act in a cascade-like mechanism. The resistance gene gyrB(R), encoding an aminocoumarin-resistant gyrase B subunit, may initially be co-transcribed with the genes from novH to novW. However, when the gyrase inhibitor novobiocin accumulates in the cultures, gyrB(R) is transcribed from its own promoter. Previous work has suggested that this promoter is controlled by the superhelical density of chromosomal DNA.

Genetic engineering of antibiotic biosynthesis for the generation of new aminocoumarins

The aminocoumarin antibiotics novobiocin, clorobiocin and coumermycin A(1) are inhibitors of gyrase and highly effective antibacterial agents. Their biosynthetic gene clusters have been cloned from the respective Streptomyces producer strains, and the function of nearly all genes contained therein has been elucidated by genetic and biochemical methods. Efficient methods have been developed for the genetic manipulation and the heterologous expression of the clusters, and more than 100 new derivatives of these antibiotics have been generated by metabolic engineering, mutasynthesis and chemoenzymatic synthesis, providing a model for the power of genetic and genomic methods for the generation of new bioactive compounds.

Aromatic prenylation in phenazine biosynthesis: dihydrophenazine-1-carboxylate dimethylallyltransferase from Streptomyces anulatus

The bacterium Streptomyces anulatus 9663, isolated from the intestine of different arthropods, produces prenylated derivatives of phenazine 1-carboxylic acid. From this organism, we have identified the prenyltransferase gene ppzP. ppzP resides in a gene cluster containing orthologs of all genes known to be involved in phenazine 1-carboxylic acid biosynthesis in Pseudomonas strains as well as genes for the six enzymes required to generate dimethylallyl diphosphate via the mevalonate pathway. This is the first complete gene cluster of a phenazine natural compound from streptomycetes. Heterologous expression of this cluster in Streptomyces coelicolor M512 resulted in the formation of prenylated derivatives of phenazine 1-carboxylic acid. After inactivation of ppzP, only nonprenylated phenazine 1-carboxylic acid was formed. Cloning, overexpression, and purification of PpzP resulted in a 37-kDa soluble protein, which was identified as a 5,10-dihydrophenazine 1-carboxylate dimethylallyltransferase, forming a C-C bond between C-1 of the isoprenoid substrate and C-9 of the aromatic substrate. In contrast to many other prenyltransferases, the reaction of PpzP is independent of the presence of magnesium or other divalent cations. The K(m) value for dimethylallyl diphosphate was determined as 116 microm. For dihydro-PCA, half-maximal velocity was observed at 35 microm. K(cat) was calculated as 0.435 s(-1). PpzP shows obvious sequence similarity to a recently discovered family of prenyltransferases with aromatic substrates, the ABBA prenyltransferases. The present finding extends the substrate range of this family, previously limited to phenolic compounds, to include also phenazine derivatives.

Prenyl transfer to aromatic substrates: genetics and enzymology

Aromatic prenyltransferases catalyze the transfer of prenyl moieties to aromatic acceptor molecules and give rise to an astounding diversity of primary and secondary metabolites in plants, fungi and bacteria. Significant progress has been made in the biochemistry and genetics of this heterogeneous group of enzymes in the past years. After 30 years of extensive research on plant prenylflavonoid biosynthesis, finally the first aromatic prenyltransferases involved in the formation of these compounds have been cloned. In bacteria, investigations of the newly discovered family of ABBA prenyltransferases revealed a novel type of protein fold, the PT barrel. In fungi, a group of closely related indole prenyltransferase was found to carry out aromatic prenylations with different substrate specificity and regiospecificity, and to catalyze both regular and reverse prenylations.

Aminocoumarins mutasynthesis, chemoenzymatic synthesis, and metabolic engineering

The aminocoumarin antibiotics novobiocin, clorobiocin and coumermycin A(1) are formed by different Streptomyces strains and are potent inhibitors of bacterial gyrase. Their biosynthetic gene clusters have been analyzed in detail by genetic and biochemical investigations. Heterologous expression of these gene clusters by site-specific integration into the genome of the fully sequenced host Streptomyces coelicolor A3(2) readily results in an accumulation of the antibiotics in yields similar to the wildtype strains. In recent years, the aminocoumarins have developed into a model system for the generation of new antibiotics by genetic methods. Prior to heterologous expression in S. coelicolor, cosmids containing the complete biosynthetic clusters can be manipulated in Escherichia coli by lambda RED-mediated recombination, creating single or multiple gene replacements or gene deletions. Thereby, mutant strains are generated which are blocked in the synthesis of certain intermediates or in specific tailoring reactions. For instance, mutasynthetic experiments can subsequently be carried out to generate aminocoumarin antibiotics that contain modified acyl moieties attached to the aminocoumarin core, and chemoenzymatic synthesis can be employed for the acylation of the deoxysugar moiety of structural analogues of the aminocoumarin antibiotics. Metabolic engineering-the combination of gene deletions and foreign gene expression via replicative expression vectors-can be used to generate further structural variants of these antibiotics. These methods can be combined, allowing the generation of a wide variety of new compounds. This chapter may provide general pointers for the use of genetic methods in the generation of new antibiotics.