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DEVELOPMENT OF AN ENVIRONMENTALLY FRIENDLY, COST EFFECTIVE PROCESS FOR THE PRODUCTION OF CYTOVENE® ANTIVIRAL AGENT
Gary B. Semones, Ph.D., P.E., Sam L. Nguyen, Ph.D., J.D., Yeun-Kwei Han, Ph.D., Eric Lodewijk, Ph.D., and George Schloemer, Ph.D.
Boulder Technology Center Roche Colorado Corporation th 2075 North 55 Street Boulder, CO 80301-2880
[email protected] [email protected]
Key words: Cytovene, Ganciclovir, batch process modeling, Responsible Care.
Prepared for Presentation at the 2000 Spring AIChE Meeting, Atlanta, GA March 5-10.
Copyright © Gary Semones, Sam L. Nguyen, Yeun-Kwei Han, Eric Lodewijk, and George Schloemer 1/2000, Roche Colorado Corporation Unpublished
AIChE shall not be responsible for statements or opinions contained in papers or printed in its publications.
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Introduction
One of the missions of the Roche Colorado Corporation’s Boulder Technology Center is to develop second generation processes for the production of pharmaceutical compounds. The Tech Center, as it is known, develops new chemistry and implements state-of-the-art engineering technology to create less expensive processes, reduce cycle times, improve throughput, and minimize the impact to the environment. Roche Colorado Corporation is a Responsible Care company. The Chemical Manufacturers Association (CMA) launched the Responsible Care program in 1988 to respond to public concerns about the manufacture and use of chemicals. Part of the guiding principles of the Responsible Care program is to make health, safety, the environment, and resource conservation critical considerations for all new and existing products and processes.
Scientists and engineers at the Tech Center perform second
generation process development in accordance with the guiding principles of the CMA’s Responsible Care program. This paper discusses the development of a novel environmentally friendly process (i.e.,
“Green
pharmaceutical
chemistry”) for
the
for
the
treatment
production of
of
Cytovene
cytomegalovirus
(ganciclovir),
(CMV)
retinitis
a in
immunocompromised patients. The second generation Guanine TriEster (GTE) Process reduced air emissions by
∼66%
and liquid/solid waste generation by
∼89%.
The
increases in raw material costs were more than offset by improvements in equipment utilization as reflected in the overall process throughput.
Thus the objectives of the
Responsible Care program were met while also meeting and exceeding the profitability and production capacity demands of the corporation.
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Background
Process research and development activities toward establishing second generation processes for the reduction of manufacturing costs for bulk pharmaceutical drug substances has been a central goal for process chemists and engineers at the Roche Boulder Technology Center.
The synergistic application of the basic principles of
process modification through the development of novel reaction pathways, in conjunction with efficient chemical engineering principles have been realized in the successful demonstration of the new Guanine TriEster (GTE) Process for the manufacture of Cytovene®. Cytovene® is a potent antiviral agent for the treatment of cytomegalovirus (CMV) retinitis infections in immunocompromised patients, including patients with AIDS. Patients who are particularly at risk for developing CMV infections include those with AIDS and patients who are recipients of solid tissue transplants.
O N
HN H 2N
N
N
OH O
Ganciclovir (CYTOVENE)
3
OH
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This presentation provides an ex post facto detailed analysis and comparison of the relative manufacturing efficiencies between the first generation Persilylation Process and the second generation GTE Process for the commercial manufacture of ganciclovir using a commercial modeling software program (Batch Plus from Aspen Technology, Inc.). Specific parameters calculated for documenting the relative efficiencies of the two processes include estimated costs, energy usage, mass and energy balances, estimated cycle times, air emissions, and solid and liquid waste streams . The qualitative results of this evaluation revealed that significant process efficiencies resulting in cost reduction and reduced global environmental impact can be attained from the judicious chemical modification to a convergent synthesis, the selective application of the principles of molecular conservation, and application of creative engineering principles for optimal process design. In 1974, scientists at Wellcome discovered the potent antiviral agent acyclovir (Zovirax®) for the treatment of various viral infections including herpes viruses HSV-1 and HSV-2.
The corresponding bis-hydroxy homologue, generically known as
ganciclovir (Cytovene®), which was developed by Syntex, proved to be more effective in inhibiting cellular DNA polymerase that is associated with viral infections. Subsequent to these early discoveries, significant advances have been made to prepare various guanosine analogs for the inhibition of viral activities related to both RNA and DNA viruses (Robins, 1986). The major synthetic challenge encountered with the preparation of ganciclovir and related guanosine derivatives involve the selective N-alkylation of guanine and its derivatives.
In particular, the selective alkylation reaction of guanine affording the
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desired N-9 product over the N-7 isomer is an exceptional challenge. This is due in part to the lack of stereoelectronic differentiation between the N-9 and the N-7 centers and the known kinetic isomerization and thermodynamic equilibration of the corresponding alkylated product mixtures (Singh et al., 1999). In addition, the amphoteric property and the highly insoluble characteristics of guanine in most organic solvent systems limit the application of the standard reaction conditions (Claussen and Juhl-Christensen, 1993). Many different approaches have been explored for the selective functionalization of either guanine or various guanine derivatives. Previously reported conditions for the selective alkylation reaction, that is, N-alkylation processes that afford predominantly or exclusively the desired N-9 over the undesired N-7 isomer, include the use of reaction conditions not ordinarily considered to be favored by environmentalists. In particular, the application of coupling mediators comprising in part of cesium carbonate (CsCO 3) (Kim et al., 1988) and cesium iodide (CsI) (Matsumoto et al., 1970) in chlorinated hydrocarbons such as dichloromethane, and toxic metal salts such as mercuric acetate (Hg(OAc)2) (Hrebabecky and Farkas, 1974), mercuric cyanide (Hg(CN) 2) (Kim et al., 1991), zinc chloride (ZnCl 2) (Matsumoto et al., 1970) and tin tetrachloride (SnCl 4) (Garner and Ramakanth, 1988). Incidental reaction conditions employed with the intent to increase the solubility with concomitant increase in selectivity of alkylation involving substituted guanine derivatives have included harsh alkylating conditions such as dimethylformamide (Slusarchyk et al., 1989), dimethylsulfoxide (Phadtare and Zemlicka, 1989), sulfolane (Jacobs et al, 1989), and related high boiling solvent systems. The first commercially viable process (Verheyden et al., 1986) for the manufacture of ganciclovir was developed by Roche Colorado Corporation, formerly
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known as Syntex Chemicals (see Scheme 1, Persilylation Process). The six-step process required the processing of 28 reagents and intermediates, including the purification and isolation of 5 discrete intermediates. Overall, the process afforded specification grade ganciclovir in 54% yield.
Development of the GTE Process
By early 1992, projected commercial demand for ganciclovir reached approximately 50 metric tons per year, and it was readily determined that the first generation Persilylation Process could not be used to meet these production quantities without a significant amount of capital investment in Roche’s production facilities. In 1993, the Boulder Technology Center completed the demonstration of a new and expedient process for the production of ganciclovir by 1) leveraging the basic principles of molecular conservation to minimize the creation and disposal of undesired wastes, and 2) formulating efficient process engineering design for streamlining process operation and the recycling of raw materials. The second generation Guanine TriEster (GTE) Process (Scheme 2: The GTE Process) successfully demonstrated the potential for a “one step” process for the production of ganciclovir, reduced the number of chemical reagents and intermediates from 28 to 11, eliminated the two hazardous solid waste streams, and eliminated 11 different by-products from the liquid waste streams (Lodewijk et al., 1996). In addition, of the 5 raw materials employed but not incorporated into the molecular structure of the final product, 4 were efficiently recovered in situ and reused for subsequent production batches.
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The chemistry scheme designed into the GTE Process also eliminated a potentially hazardous palladium catalyzed hydrogenation step, a procedure previously mandated by the nature of dibenzyl ether protecting group incorporated in the Persilylation Process. Furthermore, particularly critical to the success of applying the principles of molecular conservation was the judicious design of the 4-carbon triester coupling reagent, which was ultimately appended into the purine nucleus as the key N-9 substituted side chain. The triester coupling reagent was designed to incorporate simple low molecular weight functionalities, vis-a-vis an ethyl ester that, after condensation, would be readily unmasked under simple hydrolytic conditions to provide a minimal amount of innocuous by-products. In practice, the construction of the low molecular weight coupling reagent was readily accomplished using glycerol and propionic acid derivatives.
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OH
O
OH
N
HN
OH
H2N
N H
N
CH2(OMe)2 2 Bnzl-Cl
Ac2O
Ac2O p-TsOH
OBn OBn +
DMAP, PhCH3 O
MeOCH2OAc
OH
N
HN
AcNH
p-TsOH Hexanes
HMDS, Xylenes (NH4)2SO4, DMF
OBn OBn
1
1
AcO
N H
N
O
OBn
O O
O
OBn N
HN
AcNH
AcNH
O
N-7
50% Yield
N-9
N-9/N-7=10:1
OBn OBn
WASTE
H2 Pd(OH)2 /MeOH O
O N
HN
H2N
N
N
N
N
SiO2-AlO2 CH2Cl2
N
HN
N
HN
NH4OH, H2O N
N
AcNH MeOH
O
N
N O
Ganciclovir OH
OH
SCHEME 1: Persilylation Process 8
OH
OH
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Another key aspect for attaining higher reaction yields in the GTE Process was the novel design of the silylation reaction of guanine, a raw material prevalently found in animals and plants and isolated from guano (Shapiro, 1968).
Minor processing
modifications of the silylation reaction of guanine and subsequent coupling with the triester reagent gave rise to a highly regioselective alkylation product, thereby minimizing the formation of undesired by-products. Furthermore, the in situ acylation of the N-9/N-7 product mixture followed by the selective crystallization of the N-9 product provided a facile method for isolating exclusively the desired N-9 product in high chemical yield. Final deprotection to remove the esters and amide protecting groups could be readily accomplished in a single step using aqueous ammonium hydroxide, to afford the desired ganciclovir in 65% yield overall. Practical engineering designs for further streamlining the GTE Process included the in situ hydrolysis of the N-9 acylated intermediate thereby establishing a “one-step”
O
1. HMDS, TfOH EtCO2
O
H2N
O OCOEt
N
HN N
N
HN
OCOEt
N H
H2N
N
N O
(EtCO) 2O, DMAP MeOH, PhCH3 2. NH4OH, MeOH
OH Ganciclovir
Scheme 2: The Guanine TriEster (GTE) Process
9
OH
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synthesis of ganciclovir and eliminated the drying, isolation and storage of the triester intermediate. Accordingly, the crystallized pure acylated N-9 product is filtered from the reaction mixture, washed with toluene, and then the product in the filter is dissolved in aqueous ammonium hydroxide as the toluene wet cake into a reaction vessel and hydrolyzed in the same medium to afford ganciclovir. When the ammonium hydroxide hydrolysis reaction is complete, gaseous ammonia from the reaction mixture is recovered through a vacuum distillation utilizing a secondary receiver containing cold water to absorb the ammonia. The innovative method of recovering gaseous ammonia under these conditions permitted the removal of ammonia gas from the product mixture with the concurrent purification of ammonium hydroxide to be recycled in the subsequent production batches. Based on the initial projected demand for the commercial production of 50 metric tons of ganciclovir per year, the annual consumption of raw materials, waste streams and processed quantity of reaction intermediates are tabulated in Table 1.
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Table 1: Raw Material Usage, Waste Streams and Intermediates
REAGENTS Guanine HMDS CF3SO3H TriEster Reagent (RCO)2O (R=Me, Et) DMAP MeOH Toluene NH4OH(aq) Dimethoxymethane P-TsOH Dichloromethane Hexanes Benzyl Chloride Glycerol Triethylamine Pyridine Xylenes Dimethylformamide Ammonium Sulfate Silica Alumina Hydrogen gas Palladium Hydroxide N-Acetyl guanine Dibenzyl Glycerol Methomethylacetate Dibenzylglyceryl methylacetate N-Acetyl DB-ganciclovir N-Acylated ganciclovir (N-7) 3 N-Acetyl ganciclovir TOTAL PROCESSED TOTAL PROCESSED AFTER RECYCLE
1 2 3
ESTIMATED ANNUAL CONSUMPTION OR PROCESSED (Kg) PERSILYLATION PROCESS GTE PROCESS 17,800 17,800 89,000 (>90% recycl) 89,000 (>90% recycl) 356 48,060 30,000 22,960 1,780 1,167 1,600,000 114,000 (>90% recycl) 15,000 231,000 (>90% recycl) 900,000 110,000 (>70% recycl) 131,904 2,400 95,000 125,400 40,000 10,720 3,000 3,000 50,000 3,100 1,500 15,0002 7,000 3,200 2 19,094 34,300 190,000 34,290 96,364 5,000 1,430 54,000 3,577,852 635,773 3,523,437 193,858
Based on a 50 MT annual production of ganciclovir. Solid waste streams. Material in liquid waste streams.
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At the 50 metric ton annual production level, a comparison of the processed raw materials and intermediates between the two processes reveals that approximately 3.58 million kg are required in the Persilylation process while only 636 thousand kg are required in the GTE process.
Implicit in this disparity of material usage are the
significant global savings associated with the initial sourcing, production, shipping, handling, warehousing, processing and disposal of ancillary raw materials, intermediates and waste by-products.
Significantly reduced environmental impact are concurrently
achieved in the decrease in associated energy consumption and emissions in the processing of fewer raw materials ( vide infra).
Comparison of the Persilylation and GTE Processes
For purposes of this paper, the Persilylation and GTE processes were compared by means of a series of computer simulations using Aspen Technology, Inc. Batch Plus software.
While production data are available for running both processes in
manufacturing-scale equipment, the processes were run using different equipment combinations, different utilities, and at different scales. These parameter variations could potentially bias the process evaluation unfairly in favor of one process or the other. The use of Batch Plus software facilitated the modeling of the two operations assuming identical equipment and utilities, identical operations protocols, and identical scales. Output from the models was compared to process data to evaluate the accuracy of the models. Modifications were made as necessary to ensure the accuracy of the process models. Where information was either unknown or not available, the models assumed the default conditions in the modeling software. The results of the comparison of the
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Persilylation and GTE processes are shown in Table 2. Discrepancies between entries in Tables 1 and 2 are due to differences between forecast product and raw material demands (Table 1) and the processes as modeled based on final recipes (Table 2).
Table 2: Comparison of the Persilylation and GTE Processes for the Production of Ganciclovir (Basis: 1,000 kg Ganciclovir output)
Persilylation Process
GTE Process
∆%
20
9
-55.0%
38,469
16,892
-56.1%
58.0
27.2
-53.1%
Vapor emissions (kg)
2,102
712
-66.1%
Solid and liquid waste (kg)
39,990
4,315
-89.2%
Energy for heating (kcal)
3,746,110
1,985,926
-47.0%
Energy for cooling (kcal)
1,463,620
369,286
-74.8%
54%
65%
11%
49
29
-40.8%
0.35
1.27
262.9%
18,742
1,133
-94.0%
Number of Raw Materials Weight of raw materials (kg) 3
Total reactor volume required (m )
Overall process yield Cycle Time (h) 3
Throughput (kg/m -h) Fresh Solvent Utilization (kg)
As shown, both the number and weight of raw materials consumed to produce 1,000 kg of Cytovene is greatly reduced in the GTE Process versus the Persilylation Process. This is the result of several factors. The overall chemistry of the GTE Process is greatly simplified as compared to the Persilylation Process. In full-scale processing, there is a total of only five reactions and the isolation of one intermediate in the synthesis
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using the GTE Process versus eight reactions involving the isolation of four intermediates in the Persilylation Process.
Eliminating reactions and isolations reduces both the
number of reagents required and the amount of solvent consumed during filtration and washing the filter cake. In addition to simplified process chemistry, the GTE Process has an 11% increase in yield from guanine to Cytovene . The optimized chemistry and elimination of process steps are also reflected in reduced process emissions and decreased liquid/solid waste generation. This results from decreased solvent handling (e.g., solvent strip operations, filtrations, drying), process optimization to eliminate solid by-products and decrease the use of filter-aids, and also the implementation of solvent recovery operations. The GTE Process recovers over 90% of the input solvents. This is also reflected in the fresh solvent utilization entry in Table 2. Furthermore, the GTE Process requires 94% less fresh solvent that the Persilylation process. By eliminating process steps involving reactions, heat-ups, cool-downs, solvent strips, and drying operations, the utility demand is significantly decreased for the GTE Process. The heating demand is down 47% and the cooling demand decreased by
∼75%.
This represents not only a cost savings for production, but decreases fossil fuel demands and the corresponding emissions resulting from energy production. Raw materials for the GTE Process were about 2.5 times as expensive as in the Persilylation Process. However, this increase was offset by several factors including those discussed above. The factor that made it financially attractive to pursue the second generation chemistry was the increase in process throughput. This is an indicator of the efficiency of equipment and time utilization and is measured in mass of product produced
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per cubic meter of total vessel volume per hour of process cycle time. For the GTE 3
3
Process the throughput was 1.27 kg/m -h versus 0.35 kg/m -h for the Persilylation Process – an increase of approximately 263%. In addition to the environmental benefits and cost effectiveness of implementing the second generation GTE Process for Cytovene , there are secondary, less quantifiable benefits as well.
By streamlining operations in the GTE Process, the isolation and
storage of several process intermediates were eliminated as well. This decreases the “Goods in Process” inventory and frees valuable warehouse space that could be used for other demands or be eliminated entirely. Another benefit is a reduction in burden on the environment resulting from decreased raw material consumption.
The more efficient use of raw materials and
solvents decreases the demand on natural resources, reduces the consumption of fossil fuels, and decreases emissions and waste streams produced by suppliers of raw materials.
Conclusion
The second generation Guanine TriEster (GTE) Process for the manufacture of ganciclovir, a potent antiviral agent for the treatment of cytomegalovirus (CMV) retinitis infections, was successfully developed at Roche Colorado Corporation to meet the commercial demand of the market. By leveraging the synergistic collaborative efforts of developing new and convergent chemical pathways with creative engineering principles, the GTE Process permitted Roche to meet the significantly increased commercial demand for ganciclovir and, at the same time, significantly reduced the potentially significant environmental impact associated with large scale manufacture. By using a commercial
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process modeling program, an accurate assessment of the qualitative efficiencies between the first generation Persilylation and GTE processes may be determined.
Second
generation pharmaceutical process development resulting in environmentally responsible processes is indicative of Roche Colorado Corporation’s commitment to the environment and to the Chemical Manufacturers Association Responsible Care program.
Acknowledgements:
The initial drive and creativity of the original co-inventors of the GTE Process described in U.S. patent 5,565,565, including Drs. Eric Lodewijk, Yeun-Kwei Han, and George Schloemer are gratefully acknowledged. Implementation of the GTE Process for commercial production would not have been possible without the full support of our friends and colleagues at the Boulder Technology Center and Roche Colorado Corporation. This work was supported by Roche Colorado Corporation, a subsidiary of Roche Holding Ltd.
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References
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