Patent Description:
Injection devices for setting and dispensing a single or multiple doses of a liquid medicament are as such well-known in the art, e.g. <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> or <CIT>. Generally, such devices have substantially a similar purpose as that of an ordinary syringe.

Injection devices, in particular pen-type injectors have to meet a number of user-specific requirements. For instance, with patient's suffering chronic diseases, such like diabetes, the patient may be physically infirm and may also have impaired vision. Suitable injection devices especially intended for home medication therefore need to be robust in construction and should be easy to use. Furthermore, manipulation and general handling of the device and its components should be intelligible and easy understandable. Moreover, a dose setting as well as a dose dispensing procedure must be easy to operate and has to be unambiguous.

Typically, such devices comprise a housing including a particular cartridge holder, adapted to receive a cartridge at least partially filled with the medicament to be dispensed. Such devices further comprise a drive mechanism, usually having a displaceable piston rod which is adapted to operably engage with a piston of the cartridge. By means of the drive mechanism and its piston rod, the piston of the cartridge is displaceable in a distal direction or dispensing direction and may therefore expel a predefined amount of the medicament via a piercing assembly, which is to be releasably coupled with a distal end section of the housing of the injection device. The medicament to be dispensed by the injection device is provided and contained in a multidose cartridge. Such cartridges typically comprise a vitreous barrel sealed in distal direction by means of a pierceable seal and being further sealed in proximal direction by the piston. With reusable injection devices an empty cartridge is replaceable by a new one. In contrast to that, injection devices of disposable type are to be discarded when the medicament in the cartridge has been dispensed or used-up.

Existing and commercially distributed pen-injectors may be equipped with cartridges providing a predefined volume for a liquid medicament, e.g. <NUM>. The injection devices are typically configured to provide user-operated variable setting of a dose and subsequent dispensing or delivery thereof. Typically, a dose can be selected and dispensed in discreet steps of e.g. one or several International Units (IU). Most injection devices on the market are configured and designed for use of grown up patients.

In the field of pediatric treatment it is desirable to reduce the dose of medicaments and to administer non-integer amounts of IU to a patient. This requires smaller dose increments to be selectable. It is therefore desirable to set and to dispense doses having a size of less than one IU, e.g. of half units. Hence, it is desirable to provide dose setting with increments of half units or less in order to increase dosing accuracy and to allow for a finely segmented, precise setting and dispensing of doses of a particularly small size.

In order to meet these requirements existing injection devices require modifications.

It is therefore an object of the present invention to provide a rather simple approach to enable setting and dispensing of doses of reduced size, in particular to provide setting and dispensing of half units or other non-integer units of a liquid medicament by making use of an existing drive mechanism of an injection device. The present invention should provide modifications to an existing injection device, which modifications are simple and cost efficient to realize.

In a first, not claimed aspect an injection device for delivery of a liquid medicament is provided. The injection device, typically of pen-injector type comprises a housing to accommodate a cartridge and a drive mechanism. The cartridge also belonging to the injection device comprises a tubular-shaped barrel filled with a liquid medicament. The cartridge is sealed in a proximal direction by a piston that is displaceably arranged inside the barrel. At a distal end the cartridge is typically provided with a pierceable seal acting and serving as a septum. For delivery of the liquid medicament, in particular for injecting of a dose of the liquid medicament into biological tissue the distal seal of the cartridge is to be pierced and penetrated by a double-tipped injection needle in such a way, that a proximally directed tapered or tipped end of the injection needle is located in the interior volume of the cartridge. Then, by exerting a distally directed pressure onto the piston the piston is advanced in distal direction by a predetermined distance thereby expelling a predefined amount of the medicament through the injection needle.

The injection device further comprises a drive mechanism having a piston rod to advance in a distal direction. The piston rod is configured to abut against a proximally facing surface of the piston of the cartridge. The drive mechanism is typically configured to provide setting and dispensing of doses of variable size. The drive mechanism typically comprises a dose setting member and a dose dispensing member that provide individual and variable setting and dispensing of a dose by the end user or patient. However, the injection device is not limited to drive mechanisms providing variable and user settable dose dispensing but the invention may also relate to so called fixed dose injection devices that provide repeated delivery of a dose of the medicament of constant size.

The drive mechanism of the injection device is configured to advance the piston rod in distal direction by a distance z1 in order to dispense a reference volume V1 of the medicament when operably engaged with a reference cartridge having a reference diameter D1.

Hence, the drive mechanism may be configured for standard sized cartridges, e.g. having an effective filling volume of about <NUM>. Dose setting and dose dispensing as indicated to the user, e.g. by a dose indicating member or dose indicating sleeve of the drive mechanism, is configured and adapted in accordance with the dispensing rate of the medicament arising when the piston is displaced in axial direction by a predefined distance. Depending on the geometry, in particular depending on the size of the cross-section of the tubular-shaped barrel of the cartridge a displacement z1 of the piston corresponds to the ejection of a volume V1 that is given by a multiplication of the axial displacement z1 with the transverse cross-sectional area of the piston of the cartridge.

In the present context the axial direction points along the longitudinal or main cylinder axis of the tubular-shaped barrel whereas the transverse direction points perpendicular to the longitudinal direction. Furthermore, the distal direction denotes the direction pointing towards an injection site of a patient whereas the proximal direction is directed towards the opposite end of the device. An injection needle is typically arranged at a distal end of the device whereas dose setting and/or dose dispensing members of the drive mechanism are typically located at the opposite proximal end of the injection device.

The cartridge actually arranged inside the housing of the injection device has a diameter D2 that is smaller than the reference diameter D1. In this way a volume V2 of the medicament being smaller than the reference volume V1 can be dispensed when the piston rod advances in distal direction by the distance z1. Hence, one aspect of the invention is particularly dedicated to replace a standard sized cartridge by a cartridge with a reduced diameter or reduced cross-section. In this way, the ejected volume per axial displacement of the piston can be effectively reduced.

Consequently, a diameter reduced cartridge can be used with a totally unaltered drive mechanism in order to dispense particularly small sized doses of the medicament. In this way, dosing accuracy can be enhanced rather easily. When replacing a standard sized cartridge by a diameter reduced cartridge the step size of discrete dose sizes to be set and dispensed by the device can be effectively reduced. Hence, with a drive mechanism originally intended to set and to dispense dose sizes of integer numbers of international units (IU) can be used straight forward to set and to dispense doses of the medicament with sizes of non-integer IU, e.g. of half-units or other fractions of one unit. For instance it is possible to set and to dispense doses of e.g. <NUM> IU, <NUM> IU, <NUM> IU, and so on.

The diameter D2 as well as the reference diameter D1 refer to inner diameters of the tubular-shaped barrel of respective cartridges. With a tubular-shaped barrel with a circular transverse cross-section the volume V1 is related to the distance z1 and to the diameter D1 by the following formula: <MAT>. The same relationship is valid for the diameter reduced cartridge with an inner diameter of D2 smaller than D1.

In order to provide a diameter reduced cartridge it would be sufficient to increase the sidewall thickness of the cartridge while keeping the outer diameter substantially constant. Then, the cartridge with a reduced inner diameter could be directly fitted into the housing of the injection device almost without any further modifications to be made to the drive mechanism or to a cartridge holder of the housing of the injection device. It may be only necessary to modify the size of a pressure piece at the distal end of the piston rod so as to match with the reduced diameter or transverse cross-section of the piston, which is equally reduced in diameter.

In other embodiments the diameter reduction of the cartridge also implies a respective geometric modification of a cartridge holder, typically forming a distal end of the housing of the injection device and being particularly configured to accommodate and to house the cartridge therein. In embodiments, wherein also the outer diameter of the cartridge is smaller than the outer diameter of the reference cartridge it is only required that an inner diameter or an inside facing geometry of a size reduced cartridge holder matches with the size reduced outer circumference of the cartridge. The outside facing appearance and geometry of such a cartridge holder can remain unchanged compared to a standard cartridge holder configured to accommodate a reference cartridge. In this way further housing components, such like a protective cap to cover the cartridge holder and to releasably engage with the cartridge holder can be left completely unaltered.

According to a further embodiment a ratio R of the diameter D2 and the reference diameter D1, namely <MAT> is smaller than <NUM> but larger than <NUM>. In this way dose size increments can be reduced up to a factor of <NUM> compared to dose increments obtainable with the present drive mechanism when operably engaged with the reference cartridge. So the ratio R just defines the size reduction of the reference volume V1 to the volume V2. The dispensed volumes V1 and V2 differ by the ratio R. Hence, <MAT>.

In this way the reduction of the ejected volume and hence the minimum dose increment or the size of discrete dose steps can be easily modified and determined on the basis of the relation of the diameter D2 in comparison to the reference diameter D1.

Generally, the diameters D1 or D2 are substantially constant over almost the entire axial elongation of the tubular-shaped barrel of the respective cartridge. Only near a distal end the barrel may comprise a radially narrowing neck portion adjacent to the distal seal. When referring to the diameters D1 and D2 reference is made to the nominal inner diameter of the proximal end or of an axial middle section of the barrel of the cartridge.

Reducing the diameter of the cartridge and hence of its barrel is also beneficial in that the piston providing a proximal seal for the barrel is also provided with a correspondingly reduced transverse diameter or cross-section. This has the further effect, that a contact surface between the piston and an inside facing portion of the sidewall of the barrel is reduced compared to the reference cartridge. In this way, frictional forces arising from the sealing engagement of the piston and the barrel of the cartridge can be effectively reduced, thereby decreasing mechanical friction for displacing the piston inside the barrel. In effect, static friction as well as dynamic friction between the piston and the barrel can be reduced thereby also decreasing a driving force to be provided by the piston rod in order to expel the medicament from the cartridge.

According to another embodiment the ratio R = n/m, with n and m being integer numbers. When the ratio R is a rational number a further modification of the drive mechanism, especially in terms of indicating the size of a dose actually set can be simplified.

According to another embodiment the ratio R substantially equals one of the following ratios: ½, ⅓, ¼, <MAT>, ¾, <MAT> or <MAT>. By making use of a ratio R of ½ the volume V2 dispensed by a displacement of the piston rod of z1 is just divided by a factor <NUM> compared to the Volume V1 otherwise dispensed with the reference cartridge. With a ratio R = ⅓ the volume V1 is just divided by <NUM>. For the purpose of indicating the size of a dose actually set making use of such rational numbers is of particular benefit. When replacing the reference cartridge by a cartridge with a ratio R = ½ the size of the dose increments can be divided by a factor <NUM>. If the drive mechanism is originally intended to provide setting of a dose with a step size of <NUM> IU implementation and use of a diameter reduced cartridge exhibiting a ratio R = ½ the dosing step size can be reduced to <NUM> IU.

According to another embodiment the drive mechanism comprises a dose indicating member with a dose indicating scale to appear in a window of the housing for indicating a size of a dose actually set, wherein the dose indicating scale is re-sized by the ratio R compared to a reference dose indicating scale that applies or which is applicable with the dose indicating member when the drive mechanism is operably engaged with the reference cartridge. Here the ratio R does not necessarily comprise a rational number but may comprise any positive number between <NUM> and <NUM>.

When replacing the reference cartridge by a diameter reduced cartridge it is also of particular benefit also to replace the dose indicating member or at least to replace a dose indicating scale provided thereon. The dose indicating member and almost all further components of the drive mechanism may be manufactured as injection molded plastic components. For replacing a dose indicating scale it may be only necessary to modify the outer appearance of the dose indicating member. It may be sufficient to replace a printed scale on the outer circumference of the dose indicating member. The dose indicating scale may be printed or adhesively attached to the outer circumference of the dose indicating member, e.g. by means of a printed label.

Therefore, replacing the reference dose indicating scale by the dose indicating scale matching to the cartridge actually in use may just require to exchange a printed outer surface of the dose indicating member or to make use of a label provided with the re-sized dose indicating scale, which label is to be adhered to the outer circumference of the dose indicating member. The dose indicating member itself, hence its geometry and its functional implementation in the drive mechanism can remain completely unaltered.

In a further embodiment the dose indicating member comprises a sleeve rotatably supported in the housing. Typically, the dose indicating member is rotatably supported in a body of the housing. The dose indicating member is typically rotatable with regard to a longitudinal axis extending through the symmetry axis of the sleeve. The sleeve of the dose indicating member is further in helical threaded engagement with the housing. The dose indicating scale is hence helically-shaped and is provided on an outer circumference of the sleeve. In other words, the consecutive numbers or symbols representing the size of a dose are provided on the outer circumference of the sleeve in a helical manner so that during dose setting or dose dialing a series of increasing or decreasing numbers shows up in a window of the housing.

In another embodiment the housing comprises at least a body and a cartridge holder connected to the body. The body accommodates the drive mechanism and the cartridge holder accommodates a cartridge. Typically, the cartridge holder is either transparent or comprises at least one window to provide visual inspection of the cartridge and its content. For a mass manufacturing process it may be of particular benefit to provide a cartridge holder preassembly constituted by the cartridge holder with the cartridge arranged therein and to further provide a body preassembly constituted by the body with the drive mechanism assembled therein or thereto. In a final step of assembly, which could be also denoted as 'marriage' the two preassemblies are just connected and mutually interlocked so that the injection device is ready to use or almost ready to use.

The injection device may be configured and designed as a disposable device that is intended to be discarded in its entirety once the content of the cartridge has been expelled. With a disposable device the cartridge holder is typically irreleasably connected to the body which means that a detachment of cartridge holder and body after an initial connection thereof implies a destruction of at least one of cartridge holder or body.

Apart from that it is also conceivable that the injection device is configured as a reusable device, wherein the cartridge holder and the body are releasably connectable. With a reusable device the drive mechanism is further provided with a reset function so as to return the piston rod into an initial axial position during replacement of a cartridge.

According to a further embodiment the cartridge holder comprises a proximal connecting end with an insert section to engage with a receptacle of a distal connecting end of the body. Typically, the cross-section and geometric shapes of the receptacle and of the insert portion mutually match in such a way, that a positive interconnection of receptacle and insert portion can be obtained. Moreover, the insert section may be slidably insertable in axial direction into the receptacle.

The insert section may comprise at least one fastening element to positively engage with a complementary-shaped fastening element of the receptacle. Mutually engageable and complementary-shaped fastening elements of the insert section and the receptacle provide an axial interlock of cartridge holder and body. Typically, the insert section is slidably displaceable inside the receptacle until the mutually corresponding fastening elements of insert section and receptacle engage. Once the fastening elements of insert section and receptacle engage the insert section is axially fixed to the receptacle. Hence, upon mutual engagement of fastening elements of the insert section and the receptacle the cartridge holder is axially fixed to the body and vice versa.

Furthermore and according to another embodiment the cartridge holder comprises an annular flange section on an outside surface axially confining the insert section in the distal direction. A distal portion of the cartridge holder located distally from the flange section further comprises a diameter that is smaller than the diameter of the insert section. Hence, the cartridge holder may comprise a tapered structure towards the distal end so as to fix and to fasten the cartridge therein.

The insert section is axially confined or axially delimited by the radially outwardly extending flange section. The insert section typically comprises or forms a socket portion with an outer diameter that is at least slightly smaller than the outer diameter of the flange section. The receptacle of the body is configured to axially receive the entirety of the insert portion. Hence, the inner diameter of the receptacle matches with the outer diameter of the insert portion so that the insert portion is axially insertable into the receptacle to form the interconnection of cartridge holder and body. The axial extensions of the insert section and of the receptacle mutually match so that the flange section gets in axial abutment with a distally facing axial end face of the sidewall of the body forming the receptacle when the flange section is in a final assembly position inside the receptacle.

Moreover, the sidewall of the receptacle comprises a beveled axial end face that is complementary-shaped to a beveled abutment face of the flange portion. The beveled axial end face of the receptacle's sidewall forms an axial edge or axial end of the receptacle and faces towards the abutment face of the flange portion of the insert section. The beveled axial end face and the beveled abutment face of the receptacle and of the flange portion define an axial abutment of the cartridge holder and the body so as to limit an insert motion of the insert section entering the receptacle. By means of beveled faces the mutual abutment of the sidewall of the receptacle of cartridge holder or body and the correspondingly-shaped flange section of body or cartridge holder inherently provides a tolerance compensation. An axial compression of the beveled surfaces of the flange section and the distal end face of the receptacle leads to a splaying or radial deformation of the sidewalls of the receptacle and/or insert section by way of which a mutual fastening of cartridge holder and body on the basis of positively engaging fastening elements can be facilitated.

By reducing the diameter of the cartridge holder from the proximal end towards the distal end, the proximal connecting end of a diameter reduced cartridge holder can be remain substantially unchanged or unaltered compared to a reference cartridge holder configured to accommodate a reference cartridge. By means of diameter reduced distal section the cartridge holder easily provides a radial support and a radial fixing or clamping of a diameter reduced cartridge.

According to another embodiment the cartridge holder comprises at least three axially extending inner ribs protruding radially inwardly from an inside surface of the cartridge holder. Said inner ribs are further distributed around the inner circumference of the cartridge holder.

Furthermore, they are distributed around the inner circumference of the cartridge holder. It is of particular benefit, when the inner ribs are equidistantly or equiangularly distributed or spaced about the inner circumference of the cartridge holder. The at least three inner ribs may extend substantially parallel in axial direction. They may extend across the flange section. Hence, a proximal end of the inner ribs may be located in the proximal portion of the sleeve while a distal end of the inner ribs may be located in the distal portion of the sleeve.

By means of the at least three axially extending inner ribs the flexural strength and mechanical stability of the cartridge holder can be improved. In addition, the inner ribs may provide radial support for the outer circumference of the tubular-shaped cartridge when arranged inside the cartridge holder. In this way the inner ribs effectively act and serve as a radial support structure for the cartridge. Moreover, by means of radially inwardly extending ribs a difference in the outer shape or outer diameter of the proximal portion and the distal portion of the cartridge holder can be effectively reduced. In this way, the difference of the diameter between a standard cartridge and a diameter-reduced cartridge can be compensated by the radial extension of the inner ribs and by the difference in the diameters of the proximal portion and the distal portion of the sleeve.

Typically, there are provided more than just three axially extending inner ribs. For instance, there may be provided four, five, six, seven, eight or even more inner ribs typically equally spaced in circumferential direction to provide a multiple radial support for the cartridge positioned inside the cartridge holder.

Inner ribs or a constantly reducing inner diameter of the cartridge holder in distal direction further serves to axially guide and to align the cartridge inside the cartridge holder. In this way a well-defined axial guiding and a rather precise alignment of the cartridge along the longitudinal axis of the cartridge holder can be obtained. For a smooth distally directed and repeated displacement of the piston rod in the course of subsequent dose delivery procedures it is of particular benefit when the barrel of the cartridge almost exactly co-aligns with the longitudinal extend of the piston rod. In this way a strain of buckling loads to the piston can be effectively reduced and frictional forces between the piston and the side wall of the barrel of the cartridge can be kept substantially constant over the entire displacement path of the piston inside the barrel.

Another unclaimed aspect of the disclosure relates to a cartridge for an injection device as described above. The cartridge has a tubular-shaped barrel sealed in proximal direction by a piston. The barrel is filled with a liquid medicament to be delivered or to be dispensed by interaction with the injection device. Furthermore, a ratio of an axial length I versus the diameter D2 of the tubular-shaped body is larger than <NUM>, larger than <NUM> or larger than <NUM>. Hence, the ratio <MAT> is larger than <NUM>, larger than <NUM> or larger than <NUM>. Such rather elongated and diameter reduced cartridges are of particular use to provide the intended dosing accuracy or the intended reduction regarding the size of discrete steps of doses to be set and to be dispensed.

According to a further embodiment the filling volume of the cartridge confined by the piston and the sidewall of the barrel is smaller than <NUM>, smaller than <NUM> or smaller than <NUM>. In particular, the filling volume of the cartridge may be substantially around <NUM>. In further embodiments the filling volume of the cartridge may be as small as about <NUM>.

The barrel typically comprises a tubularly-shaped vitreous material, such like glass. A size reduction of the barrel and hence of the cartridge is of particular use to increase the robustness of the cartridge against mechanical impact. A cartridge with a reduced diameter compared to the reference cartridge may inherently comprise an increased structural rigidity and mechanical stability. It may be less susceptible to fracture or cracking in the event of mechanical impact.

The invention as claimed relates to a method of increasing the dosing accuracy of an injection device. In a first step there is provided an injection device for delivery of a liquid medicament, which injection device has a housing and a drive mechanism. The drive mechanism comprises a piston rod to advance in a distal direction and to butt against a proximally facing surface of a piston of a cartridge. The injection device further has a dose setting member and a dose dispensing member to set and to dispense a dose of the medicament. Said dose may either be a fixed or predefined dose or the dose may be user-settable, i.e. the size of the dose may be individually modified by the user himself.

The drive mechanism is configured to advance the piston rod in distal direction by a distance z1 to dispense a reference volume V1 of the medicament when operably engaged with a reference cartridge having a reference diameter D1. In a second step the method of increasing the dosing accuracy comprises arranging a cartridge inside the housing instead of the reference cartridge, wherein the cartridge has a tubular-shaped barrel filled with a liquid medicament and wherein said barrel is sealed in proximal direction by a piston displaceably arranged inside the barrel. The cartridge has a diameter D2 that is smaller than the reference diameter D1. In this way setting and dispensing of a dose of the medicament of a volume V2 is enabled, which volume V2 is smaller than the volume V1 when the piston rod is advanced in distal direction by the distance z1.

The present method implies replacement of a standard cartridge by a cartridge with a reduced diameter while still making use of a substantially unaltered injection device. In particular all mechanically interacting components of the drive mechanism of the injection device can remain unaltered so that the drive mechanism provides an unaltered advancing motion of a piston rod during a dose dispensing procedure. It is only by a well-defined reduction of the diameter of the cartridge that the volume of the medicament dispensed per axial displacement of the piston rod is reduced in a well-defined way.

It may be then only required to slightly modify the geometric size of a pressure piece at a distal end of the piston rod and to eventually modify the geometry of the cartridge holder if the diameter reduction of the cartridge also reflects on the outer diameter thereof. In addition and in order to provide a correct and matching dose size indicator a dose indicating member may have to be equipped with a re-sized dose indicating scale. The dose indicating member itself and its mechanical interaction with other components of the drive mechanism can remain unaltered.

In this way dosing accuracy can be improved and the step size of discreet dose sizes can be reduced without any substantial modifications to the drive mechanism of the injection device.

In a further unclaimed aspect, the injection device is configured as a manually operable injection device, wherein a force for driving the piston rod in distal direction is entirely provided by a user. The drive mechanism may therefore comprise a dose button or a comparable dose dispensing member that is displaceable in proximal direction with respect to the housing during setting of a dose and which is displaceable in distal direction by the user during dispensing of a dose, thereby transferring a distally directed thrust or a driving torque to the piston rod.

A displacement path of the dose button or dose dispensing member is directly related to a displacement path of the piston rod. The drive mechanism may provide a linear transmission between the axial displacement of the dose button or dose dispensing member and the piston rod. Hence, the displacement path of the dose button or dose dispensing member is typically larger than the displacement path of the piston rod. But the distally directed thrust provided by the piston rod and acting on the piston of the cartridge may be larger than a thrust or force required to displace the dose button or the dose dispensing member in distal direction for inducing or conducting the injection process.

In another unclaimed aspect, the injection device comprises only cartridge at a time. The injection device and its drive mechanism is configured to operably engage with only one cartridge. The injection device is hence configured as a single-cartridge device. In particular, the cartridge holder of the injection device only receives a single cartridge filled with a medicament. In order to accommodate a cartridge with a reduced diameter in comparison to a reference cartridge the inner diameter of the cartridge holder is also reduced compared to a reference cartridge holder configured to accommodate a reference cartridge.

In a further unclaimed aspect the drive mechanism comprises a single piston rod to engage with the single cartridge accommodated in the injection device. Hence, the injection device only comprises a single drive mechanism.

Hormones are for example hypophysis hormones or hypothalamus hormones or regulatory active peptides and their antagonists as listed in Rote Liste, ed. <NUM>, Chapter <NUM>, such as Gonadotropine (Follitropin, Lutropin, Choriongonadotropin, Menotropin), Somatropine (Somatropin), Desmopressin, Terlipressin, Gonadorelin, Triptorelin, Leuprorelin, Buserelin, Nafarelin, Goserelin.

Antibodies are globular plasma proteins (~150kDa) that are also known as immunoglobulins which share a basic structure. As they have sugar chains added to amino acid residues, they are glycoproteins. The basic functional unit of each antibody is an immunoglobulin (Ig) monomer (containing only one Ig unit); secreted antibodies can also be dimeric with two Ig units as with IgA, tetrameric with four Ig units like teleost fish IgM, or pentameric with five Ig units, like mammalian IgM.

Further, it is to be noted, that any reference numerals used in the appended claims are not to be construed as limiting the scope of the invention.

In the following, an embodiment of the invention is described in detail by making reference to the drawings, in which:.

The injection device <NUM> as shown in <FIG>, <FIG> is configured as a pen-type injector. It comprises an elongated housing <NUM> extending in a longitudinal or axial direction. Towards a distal direction <NUM> the injection device <NUM> comprises a distal housing component denoted as cartridge holder <NUM>. In the opposite longitudinal direction the housing <NUM> comprises a second housing component denoted as body <NUM>. Both housing components, namely cartridge holder <NUM> and body <NUM> are of tubular and elongated shape. The cartridge holder <NUM> is configured to accommodate a cartridge <NUM> comprising a tubular-shaped barrel <NUM> and being filled with a liquid medicament <NUM>. At a distal end the cartridge <NUM> comprises a pierceable seal <NUM> typically comprising a pierceable septum of an elastomeric material.

At the opposite proximal end the cartridge <NUM> is sealed by a piston <NUM> slidably arranged inside the barrel <NUM> of the cartridge <NUM>. For dispensing of a dose of the liquid medicament <NUM> the cartridge holder <NUM> comprises a threaded socket <NUM> at its distal end to receive a correspondingly threaded needle assembly with a double-tipped injection needle. A proximal tipped end of the injection needle of the needle assembly, which is presently not illustrated, is configured to pierce the distal seal <NUM> of the cartridge <NUM> thereby gaining access to the interior of the cartridge <NUM>. The distal end of the injection needle is then configured to puncture biological tissue to deliver the medicament. For medicament delivery the piston <NUM> is to be displaced in distal direction <NUM> under the action of a distally advancing piston rod <NUM> of a drive mechanism <NUM> of the injection device <NUM>. The drive mechanism <NUM> is accommodated and fixed in the body <NUM> of the injection device <NUM>.

The cartridge holder <NUM> and the body <NUM> are to be interconnected by means of a positive connection as it is explicitly shown in <FIG>. The cartridge holder <NUM> comprises a proximal connecting end <NUM> to irreleasably interconnect with a distal connecting end <NUM> of the body <NUM>. The cartridge holder <NUM> and the body <NUM> are interconnectable in an interleaved or nested way. In the presently illustrated embodiment the proximal connecting end <NUM> of the cartridge holder <NUM> comprises an insert section <NUM> axially confined in distal direction <NUM> by a radially outwardly extending flange section <NUM>. The distal connecting end <NUM> of the body <NUM> comprises a receptacle <NUM> to axially receive the insert section <NUM> of the cartridge holder <NUM>. The outer diameter of the insert section <NUM> exactly matches with the inner diameter of the receptacle <NUM> so that the insert section <NUM> can be inserted into the receptacle <NUM> by means of a sliding motion in proximal direction <NUM> relative to the body <NUM>.

The sidewall <NUM> of the receptacle <NUM> comprises a beveled axial end face <NUM> that forms a distal end of the body <NUM>. The flange section <NUM> on the outer surface <NUM> of the cartridge holder <NUM> comprises a complementary-shaped beveled abutment face <NUM> featuring a geometric shape that matches with the shape of the beveled axial end face <NUM> of the sidewall <NUM>. As it is shown in <FIG> the beveled abutment face <NUM> faces in proximal direction <NUM> whereas the beveled axial end face <NUM> faces in distal direction <NUM>.

The flange section <NUM> divides the cartridge holder <NUM> into a distal portion 29a and a proximal portion 29b. Distal and proximal portions 29a, 29b are separated by the flange section <NUM> having an annular shape and extending around the entire circumference of the cartridge holder <NUM>. A diameter of the proximal portion 29a is smaller than a diameter of the proximal portion 29b. The proximal portion may coincide with the insert section <NUM> or may define the insert section <NUM> of the cartridge holder <NUM>. In this way, the outer shape of the cartridge holder comprises a radially stepped down portion towards the distal end thereby adapted to the reduced diameter of the cartridge <NUM> located therein.

In order to irreleasably interconnect the cartridge holder <NUM> and the body <NUM> in a final assembly or final fastening position F there are provided mutually corresponding fastening elements <NUM> and <NUM> on the inside of the receptacle <NUM> and on the outside of the insert section <NUM>. The body <NUM> comprises numerous fastening elements <NUM> radially inwardly extending from the inside of the sidewall <NUM> of the receptacle <NUM>. The fastening elements are configured as snap features or snap protrusions extending radially inwardly from the sidewall <NUM> of the receptacle <NUM>.

There are provided several e.g. four fastening elements <NUM> arranged along the inner circumference of the sidewall <NUM> of the receptacle <NUM>. The fastening elements <NUM> are arranged near a flange-like threaded support <NUM> having a central through opening <NUM> through which the threaded piston rod <NUM> extends. The support <NUM> extends substantially perpendicular to the axial direction and confines the receptacle <NUM> in proximal direction <NUM>. The support <NUM> effectively divides the body <NUM> into a distal interface section formed by the receptacle <NUM> and a proximal section to accommodate the mechanical components of the drive mechanism <NUM>.

The fastening elements <NUM> provided on the inside of the sidewall <NUM> of the receptacle <NUM> comprise radially inwardly extending protrusions having a beveled section 35a facing in distal direction <NUM> and extending radially inwardly from the sidewall <NUM> to the crest of the protruding fastening element <NUM> to axially abut with a complementary shaped recessed fastening element <NUM> of the cartridge holder <NUM>. The fastening element <NUM> comprises a radially extending recessed portion terminated in proximal direction <NUM> by a radially extending abutment section to axially abut with the fastening element <NUM>. When the cartridge holder <NUM> and the body <NUM> are arranged in a final assembly configuration or fastening position F an axial interlock <NUM> between the fastening elements <NUM>, <NUM> and hence between the cartridge holder <NUM> and the body <NUM> is established.

The proximal end of the cartridge holder <NUM> comprises a beveled edge <NUM> at its outer circumference that engages with the beveled section 35a of the protrusion <NUM> as the insert section <NUM> is urged in proximal direction <NUM> into the receptacle <NUM>. The beveled edge <NUM> is at least located in an angular position on the proximal end of the insert section <NUM> that matches with an angular position of the respective fastening element <NUM>. The beveled edge facilitates mutual assembly and induces an elastic deformation of both, the sidewall <NUM> of the receptacle <NUM> and of the insert section <NUM>. The mutually corresponding fastening elements <NUM>, <NUM> of cartridge holder <NUM> and body <NUM> are subject to tensile stress and to compressive stress during insertion of the insert section <NUM> into the receptacle <NUM>, respectively.

Since the outer diameter of the insert section <NUM> matches with the inner diameter of the receptacle <NUM> an insert and fastening procedure requires elastic deformation of the housing component's cartridge holder <NUM> and body <NUM> due to the shape of the mutually corresponding fastening elements <NUM>, <NUM>. The housing components, cartridge holder <NUM> and body <NUM> are typically single pieced and are made by way of injection molding of a thermoplastic material.

During mutual assembly, the receptacle <NUM> and its sidewall <NUM> experiences a radially outwardly directed load or stress leading to tensile forces in circumferential direction inside the sidewall <NUM>. Correspondingly, the insert section <NUM> experiences a radially inwardly directed pressure leading to compressive stress in circumferential direction inside the insert section <NUM>. Since thermoplastic materials are more sensitive to tensile than to compressive stress it is of particular benefit, that weakening recessed structures are provided in the insert section of the cartridge holder <NUM>. The radially inwardly extending protrusions of the fastening elements <NUM> of the body provide a structural reinforcement so that the sidewall <NUM> in the region of the fastening elements <NUM> is less susceptible in response to tensile loads that may arise during an assembly procedure.

The recessed fastening elements <NUM> provided in the insert section <NUM> of the cartridge holder <NUM> are configured as blind holes or pocket holes and do not completely intersect the wall structure of the insert section <NUM>. Hence, a radial depth D of the recesses <NUM> is smaller than the thickness of the sidewall <NUM> of the insert section <NUM>. Making use blind recesses or pocket holes instead of through openings also enhances and improves the mechanical stability and resistivity against mechanical loads present on the respective fastening element <NUM> during assembly. As a result a rather rigid, tight and long-term mechanically stable irreleasable connection of cartridge holder <NUM> and body <NUM> is provided.

As it is further indicated in <FIG> there are also provided inner ribs <NUM> protruding radially inwardly from an inside surface <NUM> of the cartridge holder <NUM>. The inner ribs <NUM> also extend in axial direction. They comprise a beveled proximal end <NUM>. There may be provided numerous, e.g. <NUM>, <NUM> or even <NUM> or <NUM> inner ribs <NUM> along the inner circumference of the inside surface <NUM> of the cartridge holder <NUM>. The inner ribs <NUM> may equidistantly or equiangularyl spaced in circumferential or tangential direction so as to provide a radial support for the cartridge <NUM> arranged inside the sleeve <NUM>.

By way of the radially inwardly extending inner ribs <NUM> any differences of an outer diameter of the diameter reduced cartridge <NUM> and an inner diameter of the cartridge holder <NUM> can be effectively compensated. By means of the beveled proximal ends <NUM> insertion of the cartridge <NUM> from a proximal end in distal direction <NUM> and into the hollow cartridge holder <NUM> can be facilitated. Also here a mutual radial centering and mutual radial displacement of the cartridge <NUM> relative to the cartridge holder <NUM> is obtained as the distal end of the cartridge <NUM> engages and slides along the beveled proximal ends <NUM> of the inner ribs <NUM>.

In <FIG> two different cartridges, namely a reference cartridge <NUM> and a cartridge <NUM> are illustrated, respectively. Both cartridges <NUM>, <NUM> comprise a tubular-shaped barrel <NUM>, <NUM> extending in a longitudinal direction that coincides with a cylindrical symmetry axis of the barrels <NUM>, <NUM>. In an axial distal direction <NUM> the barrels <NUM>, <NUM> are sealed by a distal seal <NUM>, <NUM>, typically comprising a pierceable septum fixed to a diameter reduced neck portion <NUM>, <NUM>. Typically, the pierceable seal <NUM>, <NUM> is fixed to a distal end of the barrel <NUM>, <NUM> by means of a crimped metal cap typically made of aluminium.

As it is further illustrated in <FIG> the pierceable seal <NUM>, <NUM> can be axially penetrated by a needle <NUM>. The needle <NUM> is typically part of a needle assembly comprising a sleeve-shaped needle hub having a fastening structure that is correspondingly-shaped to the threaded socket <NUM> of the cartridge holder <NUM>. The proximal end of the needle <NUM> is insertable through a central through opening of the cartridge holder <NUM> co-aligned with the pierceable sealing <NUM>, <NUM>. In this way the proximal tipped end <NUM> of the needle <NUM> can be positioned inside the interior volume of the cartridge <NUM>, <NUM>, thereby providing a fluid channel for the liquid medicament <NUM> located inside the cartridge <NUM>, <NUM>.

Towards their proximal end the cartridges <NUM>, <NUM> are sealed by means of a piston <NUM>, <NUM> typically made of an elastomeric material, such like a natural or synthetic rubber. The piston <NUM>, <NUM> serves as a proximal seal of the barrel <NUM>, <NUM>. It is displaceable in distal direction <NUM> under the action of the distally advancing piston rod <NUM>. For injecting of a dose of a predefined size, for instance for injecting of a dose of a reference volume V1 the piston <NUM> is displaced in distal direction <NUM> by a distance z1 as shown in <FIG>. The reference volume V1 is determined by the inner reference diameter D1 of the reference cartridge <NUM> and the axial displacement distance z1. Hence, the reference volume <MAT>.

The drive mechanism <NUM> of the injection device <NUM> is configured to set and to dispense a reference volume V1 of the medicament <NUM>. Hence, for dispensing of the reference volume V1 the drive mechanism <NUM> is configured to displace the piston rod <NUM> by a distance z1.

The drive mechanism <NUM> and hence almost the entire injection device <NUM> configured and designed for a reference cartridge <NUM> is now used with a different cartridge <NUM> as shown in <FIG>. In comparison to the reference cartridge <NUM> the cartridge <NUM> is also of tubular shape but comprises an inner diameter D2 that is smaller than the reference diameter D1. For injecting of a smaller dose compared to the reference volume V1 also here a correspondingly size reduced piston <NUM> is to be displaced in distal direction by the same distance z1. Due to the reduced diameter D2 compared to the reference diameter D1 the dispensed volume V2 is smaller than the reference volume V1. The ejected volume V2 calculates as <MAT>.

In this way a size reduced dose of the medicament <NUM> can be dispensed and injected via the injection needle <NUM> by making use of one and the same drive mechanism <NUM>. There may be situations and configurations of the drive mechanism <NUM>, where the smallest discrete displacement z1 of the piston rod <NUM> corresponds or defines a minimum reference volume V1 in the range of <NUM> IU. By reducing the diameter of the cartridge from D1 to D2 the minimum dispensable size of a dose to be dispensed can be reduced to values smaller than <NUM> IU. If for instance the area of the inner cross-section of the reference cartridge <NUM> is divided by <NUM> with the cartridge <NUM> the minimum dose size to be dispensed by means of the drive mechanism <NUM> will be divided by <NUM>. A minimum dose size as well as a discrete step size of selectable doses may then equal e.g. <NUM> IU.

As it is further shown in <FIG>, the diameter reduced cartridge <NUM> comprises a ratio of an axial length I over the diameter D2 that is larger than <NUM>, larger than <NUM> or even larger than <NUM>. Moreover, a total filling volume confined by the piston <NUM> and the sidewall of the barrel <NUM> is typically smaller than <NUM>, smaller than <NUM> or smaller than <NUM>. It may be as small as <NUM> or as small as <NUM>. Reducing the diameter of the barrel <NUM> compared to the reference barrel <NUM> is further beneficial in terms of a static or dynamic friction between the piston <NUM> and the inside facing sidewall of the barrel <NUM>. In effect, the total sealing surface on the outer circumference of the piston <NUM> is reduced compared to the piston <NUM>. In this way, static as well as dynamic friction between the piston <NUM> and the barrel <NUM> is typically smaller than dynamic and static friction of the piston <NUM> and the barrel <NUM>. In effect, dispensing forces that have to be applied to a proximal thrust receiving surface <NUM> of the piston <NUM> can be effectively reduced.

This is particularly beneficial to increase a handling comfort and ease of use of the injection device.

In <FIG> one embodiment of an injection device <NUM> is illustrated comprising a drive mechanism <NUM> that has been commercially distributed over years and which is described in detail in the following documents: <CIT>, <CIT> and <CIT>. The injection device <NUM> is of disposable type. Hence, when the medicament <NUM> contained in the cartridge <NUM> has been dispensed or used up the entire device <NUM> is intended to be discarded. Therefore, the cartridge holder <NUM> to accommodate the cartridge <NUM> is irreleasably connectable to the proximal housing component, hence to the body <NUM>. A cap <NUM> to cover the cartridge holder <NUM> extending distally from the body <NUM> is releasably interconnectable with the cartridge holder <NUM>.

The drive mechanism <NUM> comprises numerous mechanically interacting components. The flange like support <NUM> of the body <NUM> comprises a threaded through opening <NUM> threadedly engaged with a distal thread <NUM> of the piston rod <NUM>. The distal end of the piston rod <NUM> comprises a bearing <NUM> on which a pressure foot <NUM> is free to rotate with the longitudinal axis of the piston rod <NUM> as an axis of rotation. The pressure foot <NUM> is configured to axially abut against the proximally facing thrust receiving surface of the piston <NUM> of the cartridge <NUM>. During a dispensing action the piston rod <NUM> rotates relative to the body <NUM> thereby experiencing a distally directed advancing motion relative to the body <NUM> and hence relative to the barrel <NUM> of the cartridge <NUM>. As a consequence, the piston <NUM> of the cartridge <NUM> is displaced in distal direction by a well-defined distance due to the threaded engagement of the piston rod <NUM> with the body <NUM>.

The piston rod <NUM> is further provided with a second thread <NUM> at its proximal end. The distal thread <NUM> and the proximal thread <NUM> are oppositely handed.

There is further provided a drive sleeve <NUM> having a hollow interior to receive the piston rod <NUM>. The drive sleeve <NUM> comprises an inner thread threadedly engaged with the proximal thread <NUM> of the piston rod <NUM>. Moreover, the drive sleeve <NUM> comprises an outer threaded section <NUM> at its distal end. The threaded section is axially confined between a distal flange portion <NUM> and another flange portion <NUM> located at a predefined axial distance from the distal flange portion <NUM>. Between the two flange portions <NUM>, <NUM> there is provided a last dose limiting member <NUM> in form of a semi-circular nut having an internal thread matching the threaded section <NUM> of the drive sleeve <NUM>.

The last dose limiting member <NUM> further comprises a radial recess or protrusion at its outer circumference to engage with a complementary-shaped recess or protrusion at an inside of the sidewall <NUM> of the body <NUM>. In this way the last dose limiting member <NUM> is splined to the body <NUM>. A rotation of the drive sleeve <NUM> in a dose incrementing or clockwise direction during consecutive dose setting procedures leads to an accumulative axial displacement of the last dose limiting member <NUM> relative to the drive sleeve <NUM>. There is further provided an annular spring <NUM> that is in axial abutment with a proximally facing surface of the flange portion <NUM>. Moreover, there is provided a tubular-shaped clutch member <NUM>. At a first end the clutch member <NUM> is provided with a series of circumferentially directed saw teeth. Towards a second opposite end of the clutch member <NUM> there is located a radially inwardly directed flange.

Furthermore, there is provided a dose dial or dose indicating sleeve <NUM>. The dose indicating sleeve <NUM> is provided outside of the spring <NUM> and the clutch member <NUM> and is located radially inward of the body <NUM>. A helical groove <NUM> is provided about an outer surface of the dose indicating sleeve <NUM>. The body <NUM> is provided with a window <NUM> through which a part of the outer surface of the dose indicating sleeve <NUM> can be seen. The body <NUM> is further provided with a helical rib at an inside sidewall portion of an insert piece <NUM>, which helical rib is to be seated in the helical groove <NUM> of the dose indicating sleeve <NUM>. The tubular shaped insert piece <NUM> is inserted into the proximal end of the body <NUM>. It is rotationally and axially fixed to the body <NUM>. There are provided first and second stops on the body <NUM> to limit a dose setting procedure during which the dose indicating sleeve <NUM> is rotated in a helical motion relative to the body <NUM>.

A dose dial grip <NUM> is disposed about an outer surface of the proximal end of the dose indicating sleeve <NUM>. An outer diameter of the dose dial <NUM> typically corresponds to the outer diameter of the body <NUM>. The dose dial <NUM> is secured to the dose indicating sleeve <NUM> to prevent relative movement therebetween. The dose dial <NUM> is provided with a central opening.

Furthermore, a dose button <NUM> of generally T-shape is provided at a proximal end of the injection device <NUM>. A stem <NUM> of the dose button <NUM> extends through the opening in the dose dial <NUM> through an inner diameter of extensions of the drive sleeve <NUM> and into a receiving recess at the proximal end of the piston rod <NUM>. The stem <NUM> is retained for limited axial movement in the drive sleeve <NUM> and against rotation with respect thereto. A head <NUM> of the dose button <NUM> is generally circular. A skirt extends from a periphery of the head <NUM> and is further adapted to be seated in a proximally accessible annular recess of the dose dial <NUM>.

To dial a dose a user rotates the dose dial <NUM>. With the spring <NUM> also acting as a clicker and the clutch member <NUM> engaged, the drive sleeve <NUM> the spring or clicker <NUM>, the clutch member <NUM> and the dose indicating sleeve <NUM> rotate with the dose dial <NUM>. Audible and tactile feedback of the dose being dialed is provided by the spring <NUM> and by the clutch member <NUM>. Torque is transmitted through saw teeth between the spring <NUM> and the clutch member <NUM>. The helical groove <NUM> on the dose indicating sleeve <NUM> and a helical groove in the drive sleeve <NUM> have the same lead. This allows the dose indicating sleeve <NUM> to extend from the body <NUM> and the drive sleeve <NUM> to climb the piston rod <NUM> at the same rate. At a limit of travel a radial stop on the dose indicating sleeve <NUM> engages either with a first stop or a second stop provided on the body <NUM> to prevent further movement. Rotation of the piston rod <NUM> is prevented due to the opposing directions of the overall and driven threads on the piston rod <NUM>.

The last dose limiting member <NUM> keyed to the body is advanced along the threaded section <NUM> by the rotation of the drive sleeve <NUM>. When a final dose dispensed position is reached, a radial stop formed on a surface of the last dose limiting member <NUM> abuts a radial stop on the flange portion <NUM> of the drive sleeve <NUM>, preventing both, the last dose limiting member <NUM> and the drive sleeve <NUM> from rotating further.

Should a user inadvertently dial beyond the desired dosage, the pen-injector <NUM> allows the dosage to be dialed down without dispense of the medicament from the cartridge <NUM>. For this the dose dial <NUM> is simply counter-rotated. This causes the system to act in reverse. A flexible arm of the spring or clicker <NUM> then acts as a ratchet preventing the spring <NUM> from rotating. The torque transmitted through the clutch member <NUM> causes the saw teeth to ride over one another to create the clicks corresponding to dialed dose reduction. Typically, the saw teeth are so disposed that a circumferential extent of each saw tooth corresponds to a unit dose.

When the desired dose has been dialed the user may simply dispense the set dose by depressing the dose button <NUM>. This displaces the clutch member <NUM> axially with respect to the dose indicating sleeve <NUM> causing dog teeth thereof to disengage. However, the clutch member <NUM> remains keyed in rotation to the drive sleeve <NUM>. The dose indicating sleeve <NUM> and the dose dial <NUM> are now free to rotate in accordance with the helical groove <NUM>.

The axial movement deforms the flexible arm of the spring <NUM> to ensure the saw teeth cannot be overhauled during dispense. This prevents the drive sleeve <NUM> from rotating with respect to the body <NUM> though it is still free to move axially with respect thereto. The deformation is subsequently used to urge the spring <NUM> and the clutch member <NUM> back along the drive sleeve <NUM> to restore the connection between the clutch member <NUM> and the dose indicating sleeve <NUM> when the distally directed dispensing pressure is removed from the dose button <NUM>.

The longitudinal axial movement of the drive sleeve <NUM> causes the piston rod <NUM> to rotate through the through opening <NUM> of the support <NUM> of the body, thereby to advance the piston <NUM> in the cartridge <NUM>. Once the dialed dose has been dispensed, the dose indicating sleeve <NUM> is prevented from further rotation by contact of a plurality of members extending from the dose dial <NUM> with a corresponding plurality of stops. A zero dose position is finally determined by the abutment of one of axially extending edges of members of the dose indicating sleeve <NUM> with a corresponding stop of the body <NUM>.

The drive mechanism <NUM> as described above is only exemplary for one of a plurality of differently configured drive mechanisms that are generally implementable in a disposable pen-injector. Hence, one concept of the present invention, namely to modify, in particular, to reduce the diameter of a cartridge by means of a respective cartridge replacement is generally implementable with a large variety and with almost all types of injection devices comprising a cartridge filled with a medicament and being sealed by a piston for expelling of a dose of the medicament.

When replacing a standard or reference cartridge <NUM> by a diameter reduced cartridge <NUM> all mechanically interacting components of the drive mechanism <NUM> may remain unchanged or unmodified. This is also valid for the dimensions and the geometry of the dose indicating member <NUM>, presently configured as a dose indicating sleeve <NUM>. In <FIG>, one example of a dose indicating sleeve <NUM> provided with a reference dose indicating scale <NUM> is shown in comparison to the location of the dose indicating window <NUM> of the body <NUM>. As can be seen from <FIG>, the dose indicating sleeve <NUM> comprises a helical structure on its outer circumference having the shape and form of a helical groove <NUM>. Accordingly, the outer circumference of the tubular-shaped dose indicating sleeve <NUM> is provided with a reference dose indicating scale <NUM> with consecutive integer numbers <NUM>, <NUM>, <NUM>, <NUM> and so on representing integer IU.

When making use of a diameter reduced cartridge <NUM> the reference dose indicating scale <NUM> on the outer circumference of the dose indicating sleeve <NUM> has to be replaced by a resized dose indicating scale <NUM> or <NUM> as shown for instance in <FIG>, respectively. The dose indicating scale <NUM> as shown in <FIG> is applicable when the inner cross-section of the cartridge <NUM> is reduced by a factor <NUM> compared to the inner cross-section of the reference cartridge <NUM>. Consequently, the numbers of the reference dose indicating scale are divided by the factor <NUM>. The dose indicating scale <NUM> of the dose indicating sleeve <NUM> is configured to show a sequence of numbers such like <NUM>, <NUM>, <NUM>, <NUM>, and so on.

Claim 1:
A method of increasing the dosing accuracy of an injection device of pen-injector type, the method comprises the steps of:
- providing the injection device (<NUM>) for delivery of a liquid medicament having a housing (<NUM>) and a drive mechanism (<NUM>) with a piston rod (<NUM>) to advance in a distal direction (<NUM>) against a proximally facing surface (<NUM>) of a piston (<NUM>) of a cartridge (<NUM>), and further having a dose setting member (<NUM>) and a dose dispensing member (<NUM>) to set and to dispense a dose of the medicament (<NUM>), wherein the drive mechanism (<NUM>) further comprises a dose indicating sleeve (<NUM>) rotatably supported in the housing (<NUM>) and comprising a helically-shaped dose indicating scale (<NUM>, <NUM>) on an outer circumference of the dose indicating sleeve (<NUM>) to appear in a window (<NUM>) of the housing (<NUM>) to indicate setting and dispensing of a size of the dose of the medicament to a user in accordance with a dispensing rate of the medicament when the piston (<NUM>) is displaced, and wherein the drive mechanism (<NUM>) is configured to advance the piston rod (<NUM>) in distal direction (<NUM>) by a distance z1 to dispense a reference volume V1 of the medicament (<NUM>) when operably engaged with a reference cartridge (<NUM>) having a reference diameter D1,
- arranging a cartridge (<NUM>) inside the housing (<NUM>) instead of the reference cartridge (<NUM>), wherein the cartridge (<NUM>) has a tubular shaped barrel (<NUM>) filled with a liquid medicament (<NUM>) and is sealed in a proximal direction (<NUM>) by a piston (<NUM>) displaceably arranged in the barrel (<NUM>), wherein the cartridge (<NUM>) has a diameter D2 smaller than the reference diameter D1, thereby enabling setting and dispensing of a dose of the medicament of a volume V2, smaller than the volume V1 when the piston rod (<NUM>) is advanced in distal direction (<NUM>) by the distance z1, wherein a ratio R of the diameter D2 and the reference diameter D1 according to R = (D2 / D1)<NUM> is smaller than <NUM> but larger than <NUM>, and
- resizing the dose indicating scale (<NUM>, <NUM>) by the ratio R compared to a reference dose indicating scale (<NUM>) applicable with the dose indicating member (<NUM>) when the drive mechanism (<NUM>) is operably engaged with the reference cartridge (<NUM>).